If you have the room for the H100 I would get that over the H80 , the price difference is only $20 (Newegg) and since you don't know what you may do with overclocking it would be better to be prepared instead of left wanting. If the space is tight then the decision is yours to make on what to try to fit. For myself I would not want to go with the smaller one and find out later that I should have gone with the bigger one if I had the room.
I just don't like the weight on my mobo, id rather go with the water cooling on my cpu,
it offers better cooling and i think it just works better in general.
I have the antec df 85 Its got 2 120mm fans in the back so i can just pop the h100 in there and the 2 140mm Fans at the top.
I tried the Antec 620 self contained water cooler (same model as the Corsair H70 except with numerous design improvements) on that case and the radiator hit the top GFX card in my son's SLI setup. I'm not a fan of the H series units as they way way too loud for my taste....the Antec 920 ahs the same problem, 620 not bad at all .... if it fits.