I moved from a 35 MTR to the 39.5 IROK recently. I never had any issues with sidewalls with the MTRs and to be honest, the IROK sidewall seemed to be a bit thinner than the MTR when I made the switch (BTW: the MTRs still had 90% when I pulled them).
Performance wise the IROK is a much better tire in the rocks but you need to air them down fairly low to get them to stick. Beadlocks of some sort are a must.
