If I played against either of those lists, I wouldn't think "I've drawn the short straw", I'd think, "where's the rest of it?" I have more troops in my 1000 points of eldar that you do in 1500. Even a small number of troops choices (remember them?) on your opponents side will wipe your infantry off the face of the earth, leaving you with about 10 shots per turn, not enough to win a game.
I see your reasoning. However with my Iron Warriors I tend to stay at the back of the board and out of range of basic infantry guns.
However a all infantry army is the hardest for me to face I admit.
I take less troops because the lascannon ratio is lower for them, plus they cant move and fire.
3 lascannons with obliterators=210pts
3 lascannons with predator(mutated hull)=175pts
3 lascannons with troops=270pts
Remember the 40K motto,s:
Troops dont win games, heavy support and elites do.
Tactics dont win games, armys do.