Southwest for sure - I have flown all airlines you can think of out of Buffalo, and they are by far my favourite for price, schedule and no fees. If you're worried about everyone not sitting together, this rarely happens if you simply go on-line 24 hours before your flight and check in - as long as you are in A or B, you will have no problem finding 3-6 seats together. And no one has assigned seats anyhow, so if another passenger is keeping you from sitting together, I'm sure they'd move if you ask nicely. For coming home, you can check-in online at the front desk of any hotel you are near at the time, just have your confirmation number on you. They don't charge for luggage, and if you sign up for their DING program, you will be notified of cheap flights from Buffalo when available. I just flew with them in January for $72 one way. Last year in the summer we got $160 round-trip each just a month before we left. Also, if you book a flight and it goes down, you can just cancel and re-book online for the cheaper price, and the extra $$ goes in a holding account for a year to be used for future travel.
Enjoy your trip. And Buffalo airport is smaller and so easy to get to and get around at.