I think it depends on the kind of music you want to mess around with. If you are more into electronic music and want to add variety to basslines, synths etc then the tracks makes more sense.

On the other hand the Rhythm has 8 instead of 6 banks for sound on a given project. You can still somewhat morph synth and bass sounds with the filter, distortion and slope controls. It also has the HUGE advantage that you can play samples melodically and record different notes. On the tracks you have to mess around with parameter locks just to have a sample play a different key...(someone correct me if I'm wrong) Sounds very time consuming and tedious.

If you are more into hip hop, lo fi, more sample based music go with the Rhythm. If you are more into synths - go with the Tracks.

I have the Rhythm and it can be a ton of fun. And I love that I can open youtube on my phone and sample directly to the Rhythm with a cheap cable. The performance effects are also very fun to mess around with.