This is a tough one. I pretty much agree with those who said the two things make one whole. But to be honest, I get so much pleasure from both that I'd really miss either. I had some physicall problems not too long ago where the doc said lay off the ukemi for a while. I think I held out three weeks...but I did take it easy (mostly ushiro ukemi) for a while longer.
I also spent a lot of time working on improving my ukemi so my 41 year old body could deal with the keiko better. Seems to be working for now, so I don't have to make that choice yet. I hope I never do have to choose. That would stink. My thanks to Ellis Amdur and Gary Ohama on the help they gave me.