I'd probably go for two girls myself if I had a choice, just because they're a little easier to maintain (no impactions and other yuckiness to clean). However, I was pretty open to whatever piggies were up for adoption in my area and ended up with a boy. He was supposed to be a girl, but the shelter missexed him as they're more of a cat and dog place. I have a female, too, so he's probably going to get neutered so they can be together.
If you get a girl, beware of possible pregnancy. Fortunately, that shouldn't be much of a worry if you go with a reputable, knowledgeable rescue for get her from someone on Craigslist who knows the piggy they're giving away hasn't been near any males.