If you want a contrasting sound to the Rytm, maybe something digital with an architecture not primarily geared towards subtractive analogue emulation…?
Perhaps an FM machine? DX7s are still pretty cheap. Most digital synths will do some kind of FM though.
I do love my Korg Z1, but the Radias is supposedly very nice and should also have architectural flexibility too.
Nord Modular G1 or G2 are fantastic though if you want a very flexibile architecture. It’s a huge shame that they stopped making them.
if you already have your eyes on an A4 for the near future i’d look at a second hand minitaur and use the money you save to populate your iPad with some synths and other music related apps. not necessarily in this order:
Animoog
Nave
Thor
Cubesynth
Addictive
Sunrizer
Korg ipolysix
some people find it fiddly but i have got great results sequencing the minitaur and other stuff from theysys…and then of course when you get your a4 you will retain a track that you would have lost to bass duties and you can have at it on the moog with the cv output…