So, negligible controller differences aside, the RE5 demo on PS3 appears to me identical to the 360 version. This puts me in an unfamiliar position.
This is the first time that I've ever owned all current-generation consoles. It's also the first generation without a decisively technically superior platform. The PS3 may very well eclipse the 360 as developers begin to push toward both machines' limits, but that's not where things stand right now, with many multiplatform titles actually performing better on the 360, albeit, in most cases, the differences are imperceptible while I'm playing.
Assuming no content or major performance differences, it comes down to a few practical concerns. Will the game sport any online features that I'll want to take advantage of? If so, it will definitely be PS3 for me, because I'm not paying for Microsoft's wi-fi dongle and an Xbox Live subscription. Will a forced install on the PS3 version eat up my rapidly dwindling hard disk space? Considering that I've only played five PS3 titles and I'm already down to half of my 80 gigs, this is a legitimate concern.
In the case of RE5, I'm betting that the PS3 release will require install, but, on the other hand, I'll probably want that free online co-op option, even if I don't expect to take advantage of it on my first playthrough. If the game is hiding, say, a four-player Mercenaries mode, then, yeah, I'll be needing that.