It depends on the mechanic and what the game asks you to do with it. Or another way of thinking about it is, "If the inconvenience isn't too high, I (tend to) prefer realism." But it's a sliding scale.
Take inventory and encumbrance, for instance. Some games actively break me out of immersion because of weird inventory limits. Like, I can carry 9 heal potions and 9 high potions, but if I want 10 high potions instead, that's suddenly a no-go? All other things being equal, I'd prefer a game with more realistic weight/volume restrictions, so that I can pack those 10 high potions if I want. More flexibility, less breaking immersion. So far, so good.
But I tend to like games with the less realistic inventory management mechanics better in practice. Why? Because the games that do weight/volume restrictions often tend to have a lot of item drops that you can't just carry around, so if you end up needing or wanting these things you need to go on the backtrack trip from hell, which just feels terrible. You also end up dropping items to make room for new stuff, but then I always wonder if I should be backtracking to the nearest store to sell off this extra junk rather than just dropping it. Basically, the realism tends to add a level of tedium that just never seems worth it in the long run.
So give me my 9 potions and 9 high potions. I'll grumble about the arbitrary limits (and limits of 99 and 99 aren't much better; they bring other problems, usually regarding game difficulty and grinding), but I'll deal.
Still, sometimes a game will come along and do it right. I can't remember the name of the game, but there was a dungeon crawler where 1) you could sell items while in the middle of the run by sending your pet back to town, 2) you rarely needed old gear/stuff once you got better stuff, 3) you didn't need to hoard/collect a bunch of useless junk, and 4) inventory limits were never really a problem. I got tired of the game after a while and never finished it, so I can't say that this pattern held up for the entire time, but it wasn't the inventory management system that did me in.
tl;dr: I prefer realism if it's not too inconvenient, but often it is too inconvenient.