Most of the bars, clubs and restaurants that have all the nightlife in Arlington are along Wilson and Clarendon Blvd going from Ballston to Rosslyn. The center of it all along those two streets is Clarendon. You have places like the Clarendon Grille, Mister Days, Clarendon Ballroom, Whitlow's on Wilson, Eleventh, etc.
There are plenty of old and new condos right on those two streets, but you will pay for location. If you don't mind being a little bit off those two streets, the cost goes down significantly.
In Reston, Town Center is pretty much it. It only has bars and restaurants that don't have live bands or "nightlife" nighlife in the traditional sense. And there are no clubs in Reston. You do have Ned Devine's, Carpool and a few other venues in Herndon though, which is only a 10 minute drive from Reston.
Fairfax City has several college bars due to George Mason University. Plenty of the "I just turned 21 and had my first beer" going on though.
Other parts of Fairfax such as next to the Government Center have restaurants that attract a great happy hour crowd, but don't really have a nightlife scene.
The best area for nightlife is DC due to the number of options you have. Arlington is closer to DC than Reston and has plenty of venues right there for you, but you will pay a bit more for the location.
It comes down to whether or not you're willing to pay more for convenience and location. And if you are, how much more.
Let me know if I can help you out in any other way.