Why choose the one where the poo comes out?
It's different. I'm not crazy about it myself but I have done it and can appreciate it. It's certainly tighter for the guy, and some girls enjoy receiving it. I (a guy) enjoy receiving anal also - it's where your prostate is and it's a sensitive area which is enjoyable to stimulate. If you haven't tried it I strongly recommend exploring, as otherwise you're leaving a lot of potential enjoyment on the table - it's like a girl never trying playing with her clit. Girls don't have the prostate but the anus is still sensitive, and sexual enjoyment can be just as much about the idea of doing something fun than the direct stimulation - see all fetishes.
You don't usually involve the poo, unless you're into that. You clean first, inside and out. It's an extra step of prep which means it can't be as spontaneous. If you're put off by bodily fluids, or stink/bacteria, then I have bad news for you about vaginas. Mouths are also not "designed" for sex and yet you probably still enjoy blowjobs.