Hm, its very difficult. I have the Rytm for a while now and just got the OP-1 (also for 650 €). As far as I can tell the Rytm is more kind of a “serious” drum machine. Great for doing all kind of deep techno stuff with lots of control over sound shaping (reverb, delay, filter, overdrive, compressor etc.).
However the OP-1 is a great box for experimental stuff. Have a look at this tutorial:
With the sequencers both the synth and drum parts are great for abstract and glitchy beat making.
If I was you I would definitely keep the OP-1 and give it a try. You could easily sell it in a few weeks with no or only little loss, if you don’t like it.