Check the diameter of your handle bar at the stem. You have to make sure that the aerobar will fit (i.e. made for 26mm and/or 31.8mm diameter). Make sure that the length fits properly with your fore arm length. If you will be using the bike for races under UCI rules, then make sure that the bar is compliant.
Other than these, its up to how much you want to spend (i.e. generally price goes up as weight goes down) and whether carbon or alloy.
I was using a Syntace C2 clip on my Soloist (aka S1), but now have a P2C for TT, equiped with ZIPP Vuka.
2005 black Soloist Team, full Dura-Ace, Mavic Ksyrium SL
2009 UCI compliant P2C, ZIPP Vuka base and aero bars, full Ultegra, ZIPP 404