Ok I'm sure there's loads of threads dealing with what DP to buy,
More threads on double pedals than just about any other topic here on Drummerworld. It's well worth doing a search on. I'd doubt there'd be any ground left to cover on existing pedals.
but how about how to buy.
Walk into a drum store, grab a kick practice pad, grab one of each pedal, start playing them all. Getting your feet on them really is the best way to know what you like/don't like. Sure you can ask here, but you'll get a million recommendations on what other guys are playing....it won't necessarily mean that you'll like them just because they do, though.
The "feel" from a pedal can be quite personal. the best way to figure out what works for you is by trying as many as you can.
What makes one pedal different and what are the different features to look at?
There's a multitude of factors, from:
Adjustability - some can be tailored a million ways (light feel/heavy feel, longboard/shortboard, chain drive/strap drive, agressive beater attack/feathered beater attack...the list is endless) others have only basic adjustable features (footbaord angle and beater adjustment).
to Durability - some are rock solid, others are flimsy,
to Feel - chain, strap or direct drive, light feel/heavy feel.
I can't comment on price, but I can safely say that the best way to know what you like is to get your feet on them.