Warning for working Attractions, especially in MK: it gets boring verrrrry quickly. Same ride, every day, same guests, same track to walk, same buttons, etc. Although it is pretty cool to see how they work. The people I know who've worked them get sick of it quickly. There's also the problem if you get a height requirement attraction having all the parents needing to sneak their too-short kids on to the ride and dealing with that drama. However, my last job was Attractions in DAK (Kilimanjaro Safari-no height limit, and we drove the trucks!) and I loved it. Not all attractions are lame, it depends on who you are. But just be prepared for lots of repetition. And if you do go for this, it can't hurt to mention a specific place or two in your interview. They definitely wouldn't guarantee you a spot, but after three CPs, I've learned it helps.
Merch, Attraction, and Food/Bev are the most common CP roles, as I'm told, so if you put those three on your roles list you'll have an extremely good shot at getting in.
Merchandise is a great idea. You can pick up shifts in lots of locations throughout the parks, which is nice for extra hours and getting around. Yeah, I get bored really easily, so I guess my opinion is based off of that, so you don't really have to take my advice.

Haha, but Merchandise is a lot more social as well, like you have more time to actually interact and hold conversations with guests. It was one of my most favorite roles.
Food and transportation I have little to no first hand experience. However, everyone I've ever known in Quik-serv didn't like it at all. It's really fast paced and you smell bad at the end of the day and there's high demand. If you've ever worked fast food you'll kind of know what I'm talking about. I'm sure it's rewarding in other ways, too, though.
As for transportation, I've never even known anyone really who worked it, but it seems pretty fun. Especially with the boats and monorails and stuff getting to drive them around.
Another role you should look into is Vacation Planning. This role is under appreciated, in my opinion, and I wish I had known more about it before. You sit on your butt in an air conditioned room and sell park tickets to people. You learn all about the parks, and they even get you free tickets to hard ticket events and you can earn points and spend them on extra free stuff like plane tickets and DVDs from the vault and such. Definitely look into that. It also looks good in your future if you're a business or advertising type major.
Just my two cents... okay, maybe a bit more than two cents. Sorry, I tend to ramble!

Good luck, with whatever you choose! If you're a real Disney freak, you'll probably love it no matter where you get put!