Short wheelbase, or long? Two wheel or trike? So many choices!
*Short wheelbase bikes are quick to maneuver and easier to transport on cars and such. They are usually quicker to accelerate, but less stable on longer or fast downhills.
*A long wheelbase is a very stable touring platform, generally a bit heavier.
*Trikes! Two wheels in front and one in the rear may seem odd, but it is like a pedal power go-kart. Fun, very stable on slippery roads or gravel, but a little slower. Trikes weigh more, and have a third wheel to add some drag.
*Hardshell vs. mesh seats. I like the mesh seat because they support my shoulders. A hardshell is probably more efficient because it doesn't flex as you press against it, but my neck hurts on a hardshell because they are not wide enough to support my shoulders. The mesh seat also breathes more, and are much cooler.
*Along the same subject, Under Seat Steering is the best. This style of steering is mostly ignored on modern US market recumbents. It is the most relaxed style of steering, but some shun it because it is too different. There are also claims that it is dangerous (by a manufacturer that only builds Over Seat Steering models! Hmmm...) or less aerodynamic, which is always a tradeoff. Often, what is most aero, is the least comfortable. Humans, are not aerodynamic.