Might depend on what type of wheelin you do? What kind of gearing you have or are available to keep your engine in a good torque band? Crawlin in teh easy part here, it's the right gear ratio for those times you need good tire speed, without boggin the engine, to accomplish an obsticle.
I watched Tracy Jordan on 37" Krawlers school 3 very good drivers/rigs with 42 swampers on a trail once, and another time where he took a mildly built Samurai on 35's and made most on 39's and 42's look silly. There still is an element where teh driver makes a difference. It's not always about tire size. Unless Diff clearence is all your lookin for or maybe some sort of compensation for lacking in other areas?
Also the SX sidewalls are much tougher then the TSL's, the 36" TSL's are 8 ply and the 38's are 6 ply. the 38's TSL's will flex the easiest, the SX's are difficult to get to flex. IMO SX's require beadlocks to achieve the 2-3 PSI required for good sidewall flex.
TSL's are also much cheaper then the SX's, you could get your 5th (Spare) TSL for the same overall cost of 4 SX's
TSL's and SX's are horrible on the street. Krawlers are radials and have good street manners, cost alot more, 37's require a 17" rim, work better in all conditions except mud.
BTW...I run 37/15" MTR's :stirpot:
Good luck.