If you're in college, it's unlikely that you'll qualify for many of the top sign-up bonus mileage cards. Maybe the CC boards have more info about good targets for people without a long credit history but we have threads on Milesbuzz about people with reasonably high incomes, excellent credit, and a long history getting rejected for airline miles cards these days. (Granted, some of them due to churning history...)

But I'd still sign up for every applicable mileage program. When you fly, you are paying for the miles. Therefore, you should at least accept them, even with low expectations on their value.

Of your potential candidates, UA might be of more use to an infrequent traveler because they allow one-way awards. (So does AA.) Since most airfares are sold as one-ways these days, that can have meaningful value. Say your Vegas trip prices high one direction and cheap the other. 12,500 miles can get you the one-way.

Yes, you have to learn to play the availability game a bit. But even a tiny bit of flexibility I've usually found something that will work. I just did a UA one-way BOS-IAD-MCI that would have been a $375 segment on any airline. (It happened to be Boston Marathon weekend by random chance.) Not the sexiest award compared to int'l F, but hey...it kept $375 in real dollars in my pocket. (By "tiny bit of flexibility" I mean I was able to get my desired date by I had to fly at 5:45AM when my "perfect" flight would have been a more sensible midmorning departure. )