The H100 would be a better cooler overall simply because it has more radiator surface area. However, the H100 is nearly the same price as some of the entry level watercooling kits (at least, here in the US). That might be something you want to look into.
It depends on the configuration of your case: if you have room for the H 100 radiator at the top of your case the H100 is the superior cooler.
If you need to use an exhaust with the sixe of a 120 mm opening the H80 is bettter, but I would also suggest that you look at the Antec H20 920 which is similar to the H80 but also comes with a software controller for the fan speed.
I'll get the XSPC rasa, I'd probably have got it eventually anyway so why buy the closed loop thing first, it would cost more in the long run.
I'm getting an i5 3570k for those wondering why a Pentium D would need water cooling (it runs 53C max after 1hour of prime95 large ffts though)
The Rasa is a good kit, but the pump is the weakest point. I've seen some kits on FrozenCPU where they substitute a D5/MCP655 for the XSPC X2O 750 pump for a bit more, but it'll be worth it for peace of mind. It all comes down to budget though.