Hi, I chose the 430EXII mainly due to better size without too much power sacrifice. On the other hand I use it with a 7D and a shorter zoom - maybe your 5D2/70-200 combo would do better with the more powerful 580EXII. One thing about flashes that I learned is that it gets kind of addictive - once you got one you will "need" more. My suggestsion would be to go with the big one - that may delay further need.