I would say if you have lots of samples go with XO drums. Big time saver

Groove Agent 5 is drum track construction tool — it not only provides interactive control over your rhythm tracks but can provide tons of new sounds as well. Often when working with the on board Arpeggios found on the MONTAGE or MODX, you may wish that you had more flexibility from the selected phrase.

You are provided a light version of Groove Agent with your light version of Cubase. Work with it a bit. You’ll discover that you have ways to adjust complexity (when using one of its preset patterns). It is an individual choice.

The on-board Arps are inspirational — they are more inspiring than a basic click. You can adjust the “feel” of the built-in Arps using the Play FX (you have both Arp Play FX which are applied to the data prior to record; and/or you have Pattern Play FX which can be applied after you record data to the Pattern).

Once you have constructed the sections of your song in your MONTAGE... you could drag and drop your work into Cubase or you could simply *synchronize* Groove Agent with the MONTAGE, and begin to build/construct your drum track... putting fill-ins where you wish, building intensity where you wish.

I say it over and over (and particularly for those asking how to use or how to make use of a feature): there is no one way to use the Groove Agent. You can use its sounds, you can use it to trigger the MONTAGE sounds, you can combine them, you can use it to replace your MONTAGE drums, add to your MONTAGE drums, etc., etc. Figure out what your final goal is going to be... Audio playback, MIDI playback...

It’s the PhotoShop of Drum Track Construction tools... the more creative YOU are, the more interesting YOUR results.
Tons of video help on YouTube Steinberg channel!

I rarely use Cubase outside of kicking the tires when folks have Cubase issues on the forum. I did pay for full-blown Pro Cubase mainly for enhanced time-altering related features (auto resolving tempo for all points in time of a track - being able to adjust the tempo or "correct" tempo - etc). Mainly as a cheat for transcribing. The lesser versions have some features - but it's harder to do some of this stuff that Pro facilitates. I don't use Cubase, as of yet, to create/produce/assemble songs. Either in MIDI or audio domains. Another use, however, I would use Cubase as a generic MIDI editor for creating ARPs. Thus far, I've done this primarily on Montage itself. But without some of the more advanced sequencer editing features previous Motif had - this is the sort of thing now best done with the PC and Cubase works for that.

So, taking the long way around, I'm not really a candidate for answering your question. You use Cubase more "completely" than I do. Or, at least, in a way that would benefit from Groove Agent more than I do. My only use for Groove Agent may be to create different ARPs based off of Groove Agent's "patterns". However - I don't really deal with drum ARPs myself since my use is not to create songs with full instrumentation. I play live in a band with a drummer and am (mostly) not looking for drum ARPs for anything. Mostly because perhaps I can use drum patterns for exciting envelope followers or as modulation sources for side-chaining. But I'd probably not pony up just for that usage. Internal ARPs work fine for this.

I can try to put myself in your shoes. Knowing I got Cubase Pro for something I hardly do (cheat on transcriptions) - I still did it knowing I have a tool in the box in case I need to accelerate transcriptions. ... because I'm under the gun and need cheat tools. So if I transfer this to how I think you use Cubase - I would, in your shoes, probably get Groove Agent if the price was right. 50% off tends to be my general threshold for Steinberg and related software. Although this varies because I wanted to get Kontakt 12 Ultimate (not exactly Steinberg - but sort of in the same ballpark) and didn't bite when it was 50% off. The calculus is generally how much I think I'll use it graded on a curve of perceived value (where discount plays into this). If I got Kontakt that would mean I'd have to do a lot of work to get a 2nd setup going with a "dumb" MIDI controller and computer-based samples. I'm not ready to invest that much time/energy/money to get a 2nd live setup going.

If you use drum tracks in your current work - and need/want another source for material - then it seems Groove Agent has received some positive reviews in this forum before from other users. So it can increase your bag of material and seems to provide quality/professional results. If you're generally happy with built-in ARPs - I gather from user feedback that Groove Agent will provide even higher quality. Although this may be genre specific. I didn't dig into what genres the other users were dealing with. This is my disclaimer that your mileage may vary as a function of how varied your needs are.