Damn. I found myself thinking that last night. "Do I buy PGR4 if I have TDU and PGR3 already?". TDU gives me a brilliant driving experience, PGR3+Forza for the rest. I'm still downloading the demo...since 4pm and am up to 85% so far on a 19mb line :/
I'm in the same boat so to speak. I have Sega rally which is tough but fun, do I really need PGR. Plus getting it means either Halo or Rally goes as trade in.
I could just wait but patience is not a strong point.