30's would be my choice, thats what I ran with! 31's would only give you an extra 1/2" of ground clearance, but would require more horse power to turn and use more gas, and they are more expensive, 30's seam to be cheaper and more plentyfull around here.

do the 31's really take that much more power to turn than a 30? well i guess if the widths are way off yes.

I have a set of 5 truxxus m/t's for sale at the moment on 15x 8 steel wheels.

one rim has a folded lip but has held air for a couple years now.
1 tire still is basically new, 2 tires are very good shape, and the other 2 are decent shape. a little wear to the inside after running them for a few days after an accident that through my alignment out really bad.

2000 SPORTY, DEF BOY customs, mmm....lockers, mmm... gears, mmm... double transfer case, mmm... stroker