I've seen both wristed and extended arms work very well on the trail. One benifit of a wristed arm is that, when pinned on the street, you retain the handling charactoristics of the stock arm. Extended arms, IMO, tend to feel a little soft due to the increased leverage caused by the longer arm. Myself, I have a wristed arm, 5.5" WH coils, 9012's and am very pleased with the articulation derived from this combination.
For extended arms, check with Mike Ladd at
www.bentandtwisted.com. He makes a nice set of arms and has some pretty strong feelings as to why they're better than wristed. It's good to get as many informed opinions as you can then make an informed decision.
Good luck!