It's hard to put "feel" into words, but you're going to have a hard time developing organ-specific technique on any hammer action keyboard, the keys respond differently. (And the waterfall shape, per se, isn't even the most significant difference.)

I was afraid of that. I wish I could find one to try. It would be great if there was a 73 key version with 61's action. The Hammond SK Pro 73 has an organ keybed but no real-time controls like the YC.

Other alternatives:

Studiologic Numa Organ 2
Roland VR-730
Nord Stage 3 Compact

... and so on.

There's a discussion here: http://forums.musicplayer.com/ubbthreads.php/topics/3043956/re-best-73-key-waterfall-midi-controller-with-internal-sounds

Even more keys - some organists have liked the Studiologic Numa Compact 2X

The pricing approaches that of 88-key controllers so you may use the Numa Compact as a controller for a YC if you really were wanting to stick with the Yamaha.

If I buy the YC 73, what am I going to lose in organ "feel" by not having the waterfall keyboard? I need the range of 73 keys but want to develop more organ technique.

Keyboard action and opinions about keyboard action is going to get you to a point - which you seem to have already guessed - where you should really try it for yourself. “I wish I could find one to try.” — no doubt, this is best.

Since you phrase this from the standpoint of, you are not really an organist. You are asking, what are going to lose?
One answer might be nothing you may miss… missing it because you used it all the time, is very different from not knowing the playing techniques that will be very difficult to emulate on a piano-weighted action.

If you grew up exclusively playing the organ with waterfall keys, switching to a an action that is principally for piano/electric piano, would be a huge change. However, like most keyboard players, you probably grew up on piano action… so it’s the organ lexicon that is unfamiliar.

The action on piano-like-objects is going to be complex, to a dizzying degree on the actual acoustic instrument and even on electronic keyboards, you’re going to find a very sophisticated mechanism to recreate the “feel” of playing a piano.

Organ waterfall key action is nowhere’s near as complex because organ action does not have to interpret the force with which a key is pressed and turn that into loudness of the resulting Note (as piano imitating action must do). That is one huge difference in concept.

The organ vocabulary of performing gestures include slapping/bouncing on keys, smeared glissando, hit-it-and-quit-it rapid runs, etc., etc.
Just how important these will be for what your doing — really, only you can judge.

If 90% of what you do is going to be organ-related that would be one thing, but if it’s the opposite, the performing gestures that can only be done on waterfall action may not really require you get a waterfall action, only to then have to learn what they are… some may only be required in particular genres of music.

(If you need a two hand palm smear across four octaves, a very rock n roll/horror movie organ move, that’s going to be less important if your doing ballad work only).

While you’re waiting to try one for yourself, speak to someone that you know who plays organ on a regular basis, but may also play piano. Find out what performing gestures can and cannot be pulled off on a piano-weighted action… yes, it will vary per player. Yes, it will be their opinion… but perhaps they can put a kind of value on what you’ll miss or not miss. Play a waterfall keyboard if possible - you may find your answer from just doing this…

If you need the 73 key real estate, because you’re playing splits.. you can’t trade that off
If you need a wide variety of quality sounds to go along with a quality organ…
If you grew up playing a tone wheel organ, it’s easy to understand why the 61-Key has the waterfall action.
If you didn’t … find out from those you talk to what specifically they cannot pull off on the Piano-weighted action… then you can weigh just how important that is for YOU.

Hope that helps.

Remember, at the end of the day, as my good friend, Howard Massey once said: “there are only three reasons why a musicians buys a musical instrument
1) The Sound
2) The Sound
3) The Sound

Jason and Bad Mister, thank you both. I am strictly an amateur but play in a local big band. More than half of what I need is a good acoustic sound. I have a P-515 for that. The P-515 also covers the Rhodes and Wurlitzer sounds I need. It is a challenge using the P-515 live when I want to add an effect to a Rhodes sound. You can't save any presets and there is too much romping through menus. Acoustic sounds and feel are excellent. For gigs I use the P-515 with a MX61 as a top board for the few times I need an organ or other sounds.

The P-515 is a bit much to haul back and forth to rehearsal every week. The MX is great but it doesn't have the range I need, especially when the bass player is absent and I cover that part in a split. Also, the MX doesn't have the best key bed feel for the pianos. I can do some organ moves but not on the edge of the keys. I'd like to play more organ in other settings.

The appeal of the YC 73 is to have the range of notes, real time adjustments, real drawbars, and portability. I could easily take it to rehearsal every week.

I'll keep looking. Thanks for the suggestions on the other options.

the MX doesn't have the best key bed feel for the pianos. I can do some organ moves but not on the edge of the keys.

I think you may have answered your own question here. The MX action is much closer to an organ action than any hammer action is, so if you find the MX to be not quite as good as you're looking for for organ feel, it's a safe bet you'd find the YC73 noticeably worse. I think your best bet is probably pairing a waterfall organ with your hammer action piano board of choice. Sticking with Yamaha, maybe a YC61 with a CP73. Other brands have other options. One that might interest you is the Vox Continental 73 because, compared to most waterfall (organ-centric) actions, it's better than most for piano and it has 73 keys. Overall, it's not as versatile a board as a YC, but it might have a sound/feelfeature combination that happens to work for you.

I used to have the YC61. For the size it's fantastically compact for all it can do. When the YC73 came available I sold the 61 for the 73.

Now I'm not an organ player using those organ gestures so I couldn't care less. I use the organ for voice structure and building and backup.

For me the YC73 was more beefy, solid, rugged and feels very sturdy but still a nice size. I also own the SKpro which is fantastic.

I'd find a lightly used SKpro and YC61 or YC73 for a great combination or new if you can pay for it.

The authenticity of the SKpro organ with its monosynth and other capabilities is worth having.

Most organist use the double manual so stacking 2 is the best rig ever.

IF you really want to go bananas add the tiny Korg Op6 to either of these. It does FM organs too.

We only live once and then life is over. If music is your passion there is always a way to get what yo really want without buying new.

However sometimes bigbox dealers offer no interest and equal payment for 36 or 48 months. Very manageable, you just have to wait for the offer several times a year and holidays etc. .

Then I keep my gear in perfect condition and sell it for nearly what I paid for it online via Ebay and Reverb. I use that money to buy the next great model.

So once I'm within that cycle I essentially buy stuff for less than half price. If the model has been available for a while I always buy used, renewed by manufacturer or demo model and it's always been fine. I got my CP1 which was $6K at the time from a dealer on display for $2,500 stand and all. Be patient and find a great deal with a warranty too.