I own the Rytm and i had a MD UW+ MKII for several years. When it comes to percussion synthesis nothing beats the MD !! In my opinion. If you want a machine where you actually dont need to rely on samples at all to make it sound good, the MD is definitely worth it! The Rytm is not nearly as flexible as the MD is and - because the Rytm is analog - it always sounds similar every time you create something without samples. The MD has several Synthesis Methods which all sound completely different. And if you can grab a UW at a decent price you get the addition of having samples to play with and even more important: You get the actual sampling and resampling capability the Rytm doesnt have. This is priceless !! Trust me. Especially when it comes to the dubby Deep-House Chords and groovy Techhouse Grooves you can twist around by resampling them while combining them with the almost organic sounding Percussion Sounds the MD offers on some of their machines (EFM for example). I already regret that i sold my MD for the Rytm because i miss a lot on the Rytm that i got used to on the MD.
But the drawback is: The MD is more or less discontinued, so you cant expect anything new on it and if you do not have an audio interface or mixer where you have an Input Gain for a real Pre-AMP you dont get the loudest possible Mix into your computer at +4dBu and you have to post-gain which degrades the sound quality a bit. And the Rytm has the advantage that its initial sound is already quite powerful and you have Scenes and Perfomances as well as a dedicated Chromatic Mode which all are very handy - though still not perfect because especially the Scenes need to be changed in future OS Updates a bit, as untight Scene-Changes result in Audio Glitches - at least for me this happens all the time. But you have it, the Rytm (hopefully) will still receive a lot of updates and the MD does not have Performance Features in that way. Also with the Rytm you will get the Overbridge for bringing stuff into the computer much like you know it from your Virus TI already. If this is something one really needs at all for such hands on machines needs to be discovered when it’s out next year 
In any way it is not the worst idea packing an MD beneath the Rytm. The MD can still easily outperform the Rytm with its versatility and flexibility in Sound Design but lacks when it comes to the Realtime Performance Capabilities the Rytm offers. But i would highly recommend to grab the UW Version because with this you can transfer Samples between Rytm and MD and you could therefore sample your Rytm, combine with the MD and come up with amazing Drumsounds that you can then transfer back to the Rytm from the MD for Performance Use 
Always obviously depens what you need though. If you want more Synth you go better with an AK for sure. But as you already have that little Moog Beast and the “all in one for everything Synth” - the Virus i would say:
Get an MD - but get the UW-Version!! Along with the Rytm those two melt into one hell of a Drummachine no other Product on the Market could ever beat for sure!!
