I know nothing of the MX88.

I do know I just bought a MODX7 and it is blowing my socks off. It sounds glorious and gives me goosebumps.

I had an MX49 before and while it's very impressive for the price you pay, I would say that the MODX is a significant step up from the MX in terms of features, controls, Display, available sounds, the FMX sound engine, Effects etc etc.

Cheers
/Jugge

Two issues... 88 hammer action keys (available in either MX or MODX) vs. 76 non-hammer keys (MODX only, biggest non-hammer config in MX is 61 keys); and MODX vs MX.

For the actions, hammer action is better for playing piano sounds, non-hammer is better for playing organ and is often preferred for other synth sounds as well.

Key differences between MX and MODX:

MX
* 166 mb of sample data, a subset of the Motif XS sample set.
* A maximum of 4 parts at once can have effects, one effect each.
* Almost no on-board editing. There was a free computer editor, but it appears to be abandonware, leaving you with the software from John Melas (excellent, but not free).
* You'd need that editor, not just to edit the sounds, but also if you want to be able to play more than two sounds at once from the keyboard.
* No built-in sequencer, just rhythm patterns and arpeggios. It can play externally generated MIDI sequences, though, directly from another device or from a USB stick.
* Seamless sound switching is limited to switching within user-defined sets of 16 sounds.
* No specific MIDI control functions, though you can use the 16 buttons to instantly change which of the 16 channels you're transmitting on.
* Used as a 16 channel MIDI sound source, it's one sound per channel, but the external editor would allow you to put multiple sounds on some channels with none on others.
* IIRC, the knobs are fixed pots; the MODX use endless encoders

MODX
* 5.67 GB of sample data, which includes the complete Motif XS sample set and a lot more, plus a GB of space for you to load additional samples.
* A maximum of 12 internal parts at once can have effects, two effects each.
* Full on-board editing via large touchscreen
* Split/layer up to 8 parts at once from the keyboard.
* Sequencer functions
* Seamless sound switching among all sounds as long as you use only the first 4 of the 16 available Parts to create the sounds.
* Eight zone external MIDI control.
* Used as a 16 channel MIDI sound source, it's one part per channel, you would need additional hardware to assign multiple parts to the same channel.
* touchscreen patch selection
* FM synth (with its own additional polyphony)
* Motion Control enhancements

There are also differences you can tell just by looking, like the number of front panel controls, the available back panel connectivity, how big/informative the displays are.

@Kelvin

I don't know what your goals are.

This makes it difficult to advise.

What is your main use of the keyboard? Playing live? Producing/writing songs? Using as a "backing track" for something else? Hobby? Pro? Do you want the ability to edit, modify, tweak, create your own sounds - or just use what's available and already?

There's sort of a series of questions one would ask to guide through one product line or another. I didn't ask them all because one set relies on another answer.

Then "how much do you have budgeted?" would come up - and that'd help guide the rest.