Oh, by the Emperor, nooo... I don't know what they do to those poor boys to break them and turn them into those... Monsters, but I'd rather not it happen to me. And I think you've got to be fairly young, like in your teens, to be considered Spess Mehreen material. Any older, and the gene-tonics don't work, I believe. And it doesn't seem like a very fun life, sitting around in a monastery or killing things.
The only chapter I'd even consider would be the Space Wolves. At least they seem to be enjoying themselves.
No, it's the Imperial Navy all the way for me! The Emperor might rule all, but aboard the Götterdämerung, I am his second. Load the conscript catapult, ensigns! Shove more heretics into the boilers!
Or even better, one of those Rogue Traders. Working for noone but myself, selling illegal Tau farming equipment to Imperial peasants.
Oh, and one note on my dear old Imperial Guard; who do you think is winning the war whenever the Sphess Mehreens aren't around? There is only about one thousand marine chapters scattered throughout space, and most of those only have around one thousand battle brothers total, not discounting losses.
Space Marines are awesome, but they're pretty damn rare. If the Imperium's survival was entirely up to them with the IG just being there to fail and die, it'd probably be eaten within a week or so. They win alot of battles, but they can't possibly win all the battles.