What I'v always done in this situation is take two examples of common targets I expect to be shooting at, thats usually a space marine stat line and like a orc stat line and simply compare the numbers needed to achieve wounds.
I haven't played in a while so I can't give you examples but I have always looked at different choices in the game by the numbers, after all thats what its actually about.
So you just average the dice rolls needed to achieve a wound, the gun with the lowest total wins, of course you have to account for multiple shots but thats pretty easy.
My point is you should try looking at the numbers.