Several factors should be taken under your consideration.
1. Drying time. Actually, its the direct puepose to use the hand dryer.
2. Motor. It is the main parts of hand dryer machine. It determains the air speed and lifespan of a hand dryer.
3. Air speed.High speed hand dryers mainly dry hand by blowing the water away , just like an airblade. Faster air makes faster dry, but the comfortable air speed for hands is 30m/s-120m/s.
4. Economical and power consumption. They are the mainly cost of hand dyrer, we need take our cost and environmet in consideration.
Of course, excellent design and quality may be your concern too.