Well, if I were faced with that choice, it would be two ships. You have plenty of multiplayer options with the Endeavor and the Hammerhead. The Endeavor can be turned into quite a few different options on top of that with it's modularity. So I would go for smaller group play and some solo money making. It's $15 over your $400 price tag, but I feel it's worth it.
I would go with the Aquilla and a Prospector. The Aquilla should be a good choice for a small group. It has a long range, should be able to haul some cargo, and comes with a snub fighter and Ursa rover. It is fairly multipurpose and should be good if you only want to bomb around with a couple friends. The rover will also be handy to have when 3.0 is out. The Prospector should also offer some solid mindless UEE grinding. Zone out, make a few mining runs, and you should (hopefully) be able to make good income without too much effort.
Either that, or go with Bigtimes' suggestion and get a Starfarer. Personally though, I would go with strengthening your solo game. Nobody wants to hang out with us hooligans 100% of the time. Some times, you just want to zone out, farm UEE, and watch Grandma's Boy for the millionth time...