Eurogamer's Digital Foundry has their GTA V face off going live at 11am tomorrow apparently.
Instead of listening to a load of fanboy opinions it might be worth waiting to have a read.
One thing to note, a lot of people are mistakenly installing the play disk on the 360 which makes texture performance quite a bit worse.
Ultimately from what I have seen both appear pretty fantastic, and I don't think you can go wrong either way. If cost means anything 360 wouldn't need a year live sub if you won't keep the console beyond GTA.
I'm on the fence until tomorrow personally.