With the legacy airlines, I'd book round-trip. I used to book Southwest as 2 one-ways, so that if flights in one direction or the other changed I could rebook, but now they let you rebook by the direction with round trip itineraries, too.
If you are flying from Boston, consider driving to Portsmouth and taking C&J to Logan. Not sure what the bus fare will be, but parking is free, the buses run hourly and reliably, it's way easier than driving in Boston, and it makes for a shorter drive home. Another airport option is Manchester, NH. It's about 45 minutes west of Portsmouth, an easy drive, and I've scored some pretty decent prices there. If you have early morning or late night flights, they have some affordable park-n-fly hotels, and I've had luck doing Priceline for an airport hotel the night before/after and then just parked at MHT.