The 36x13.5/15 IROK specs at 36.8" OD, 6 ply, 22/32" tread depth, 11" tread width...
The 36x12.5/15 TSL specs at 36.3" OD, 8 ply, 24/32" tread depth, 10.5" tread width...
Out east in muddy rocks, the TSL (bias ply) seems to be the standard bearer for a tough tire.
I have no first hand experience with the IROK (nor do I know anyone who has) but it was designed for rocks, and is priced very close to the TSL...
Either one should do well for you...for westcoast type (dry?) rock, I bet the IROK does better, but thats just a guess...
HTH
Woody