I recently changed from a USS bike to a bike with tweener bars. I'm not sure there's much to the aero difference actually. If anything, my old USS bike had a terminal velocity going down hills several mph faster than my new bike, suggesting it may have been more aero. But hills aside, I'm much faster overall on my new bike because the position lets me generate more power, more efficiently.

To the OP: try to find a dealer and do lots of test rides. Recumbents have very varied designs, and some may suit you better than others. In general, to get the speed you're after you'll want to look for a high bottom bracket and reclined seat. Light weight would be nice too, but quickly gets expensive and a few extra lbs may not be a big deal if you're riding in mostly flat areas.