Okay, a lot of people get the 13t for trail riding, b/c it allows you to stay in a bit higher RPM given the gear and technical terrain w/ out stalling. The 15t would make it a tad harder to ride trails, but would increase your overall top end of each gear. Starting off in 2nd is fine but I wouldn't know what to say about trying it w/ the 15t. A lot of people when they drag start off in 2nd b/c usually 1st gear is more say like a granny gear, this eliminates 1 extra shift shortly after take off.
2008 700xx; Power Bomb header with power core 4 exhaust, Power Programmer III, pro armor nerfs, prm products control front and rear bumpers and full skid, 10" ITP rears, ITP Baja's fronts, RZR 2's all the way around, Uni filter, 13t sprocket, CCP steering Damper, DS450 shock swap, Dual Gasser Throttle, bar risers and Moose barkBusters. 730XX kit plus other engine mods!