Your money would be better spent on the 360, you can get a cracking deal on one at the min. The RRoD issue is way overblown IMO, the one person I know who had this issue, got a replacement console with no hassle at all. They changed a couple of bits in the manufacturing process last year to try and prevent the issue in new consoles, no issue with mine yet.
Not only is it cheaper than the PS3, and will have additional content for GTAIV, but its got some cracking games as well. Forza 2 is a worthy sim, lots of fun times to be had. Call of duty 4, Bioshock, Halo 3, PGR4, Mass Effect, the list is pretty long. The only PS3 exclusive I can think of thats notable is GT5, and its not worth paying the extra for a screenshot generator imo.
Its neither here nor there for which is better for GTA4 it seems - it plays well on 360, no noticable pop-up during gameplay, loading times may be shorter on PS3, but you only have 1 loading screen so its negliable. Not to mention the 360 will have more content added later in the year, making your money go further.
Jakg - the old ones make fantastic media servers, thats what mine does now, cheap as chips, easy to throw a 200gb HD in, reliable, and some great media software released for them. Added bonus of retro gaming when you want, emulators for them are well developed.