I've got a stock exhaust on my wolvy, and do alot of mud and dusty trail riding.. Which would be rout to go
Twin air, K&N or UNI

 

Foam air filters do better job in dusty conditions. Uni or Twin Air.

You may wish to consider a clamp on filter setup like Noss or Pro Design. The stock arrangement, which utilizes the lid to hold the filter in place, is shaky to say the least. The possibility of dirt bypassing the sealing surface and ruining your motor is real. A clamp on filter is a must in my book.