Ooh female, it's usually the best idea to have one of each.
But then again, it may not be a problem if you get a male. Of the labs I've known (and those who have owned me
) they seem to be accepting of everyone. My two couldn't care less if another dog is male or female, they just want to play! The labs I've seen at my local dog park are the same way.
But then, you know your current dog better than I do
IMO, get the one you fall in love with, and then make sure it is known in your house that YOU are the alpha dog. Then spend equal time with each one seperately from day one so jealousy/dominance issues have less of a chance to rear their ugly head. The important thing is both dogs need to bond to you first before they bond to each other. If they bond to each other first, you are in for a world of annoyance when one has to go to the vet and the other destroys your house because he was left home without his buddy. I've seen it happen. It's not pretty.
If you establish yourself as the alpha from the outset, you wont have any (or as many) dominance problems when they get older either. My current dogs knew from the outset that they were not the alpha dogs from day one and now they are the most well behaved pups on the block. This is not to say they spend their lives cowering in the corner or anything - quite the contrary actually, everyone always mentions how happy and excited they are. But they know what mom says goes and they dont even bother to try and dominate each other.
If you do get another male, I'd look into NILIF training to keep everyone's attitudes in check. It's quite helpful and very fun! Actually, I'd look into it anyway...male or female. It's excellent training.