expandable (aka mechanicals) are designed to open once it makes contact with the target. There are few styles, rear deploying(Rage broadheads), over the top (spitfires, Grim Reapers) and tip activated (Tru fire Switchblades). Maybe the bigest advantage is that they fly like a field tip. They also usually boast an impressive cutting diameter. You will find mixed reviews from people, they either love them or hate them. The reasons for loving I already mentioned. Reason to dislike are; since there are moving parts there is oppourtunity for failure(not opening when it hits the target), opening in flight and causing erratic arrow flight, and since the blades have to be forced open upon impact, you lose energy which can result in not getting a pass thru (which we all want).

Cut on contact broadheads are fixed blade broadheads where the blades run all the way to the tip and start to cut a soon as they touch the target. They are very good for penetration since all the energy goes into driving and cutting rather than deploying blades. Since they cut on contact and penetrate so well they are a very popular style among people who shoot bows that produce lower kenetic energy from either pulling less weight, having a shorter draw lengths, or shooting traditional equipment.

Personally I am a die hard muzzy shooter, they are rock solid, accurate, and if I do my part, have never let me know. Plus, as far as broadheads go, you'll be hard pressed to find a better broadhead for a better price.