A blue ray drive can only read data at a certain rate which is leagues and leagues slower than the rate a GPU will request and process the data.
Loading textures that were designed for 512MB of RAM into only 256MB is not easy to do.
You really think they're going to send you a new BluRay everytime there's a client update? Are you going to have to switch discs when you jump between add-on content in the future?
The game will run off the HDD in the PS3, just like it will on the PC. As to your RAM theory PS3 vs PC, PS3, 360, Wii... all of them have one single advantage over PC. They have a guarantee'd hardware config that developers can code specifically to. You don't need as many rescources to generate the same product. Obviously for high end PC users this is almost always a plus except for a lot of games that are multi-plat. This will be one of few that SHOULD be a better experience on the PC. I plan on playing on PC and PS3, but I've always prefered PC gaming.
Bottom line is, PS3 version will be great visually and perfect in control, PC should be visually amazing with minor control flaws... in the first part of it's lifespan.
FFXI:Sylph - Perrin 75 Hume THF; Retired (At least from my use any way)
EVE Online:ScraperX; Retired
WAR:IronClaw- Peryn SW;SkullThrone- Grymloc BO; Retired