You could just get yourself something like the EIT-1 and have them both connected to each other.

https://sites.google.com/site/companyofquail/home

Or get some rack ears and mount them in a case like the http://www.thomann.de/gb/thon_19_koffer_10_he.htm

or the http://www.thomann.de/gb/thon_19_koffer_8he_live.htm if you mount one unit raised.

Those would give you that super portable setup without having to get ride of a machine.

As for your original question, I have both the A4 and the Rytm, and really like both but I consider the Rytm slightly better at tonal melodic and bassline stuff than the A4 is at drums. Some people on the forum really like using the A4 for percussion but I always find it seems to lack some punch and used up tracks quickly unless your super creative with p-locks and sound locks. You can still only have 4 sounds going at any given time.

On the Rytm you can have 8 voices blaring at any given time, and 12 total but some share voices. The FM machines and the Toms can make some great melodic lines if you set them up with less pitch mod than drum sounds usually have. The chromatic machines on the synths currently don’t track pitch correctly but I don’t think you’d ever notice it if you weren’t mixing it with other machines. You can also load samples, including single cycle wavs to expand the tonal pallete.

The A4 does have a bunch of synth features that the Rytm doesn’t have. PWM, two filters, much bigger mod section, feedback OSC, AM, Unison, Polyphony and easy transposing. So they do compliment each other very well.