Pellets vs Leaf: I've used both, they both work well. Pellets seem to absorb less beer, but it's easier to siphon under leaf hops. I typically use pellet these days.
Type: Obviously you want an aroma hop. You also want an aroma hop that will play nicely with the ones you boiled with. Most people use ones they've boiled with. I'm a big fan of using more than one variety, i'll typically use 2-3 different hops to dry hop my APAs, IPAs, and IIPAs
Amount and time: For 5 gallon batches I like to use:
Blonde Ales, and Bitters: 0.5oz for 4-7days
American Pale Ales: 1-1.5oz for 5-7 days
American IPAs: 2-4oz for 7-14 days
Imperial IPAs: 3-6oz for 7-21 days
I amounts above vary on the recipe I'm making, and how much hop aroma I want. Whenever I dry hop for longer than 7 days, I'll split the dry hop into two or three additions. I like the idea of introducing fresh hops every additional week.