I would think the softer Iroc would be harder on axles as they provide more grip on rocks, the sx being a harder tread would slip easier.
So are you keeping your bead locks you already have?
I wonder how much haggling you can do? Never know you may be able to get either set for at least a few hun less than advertised.
Personally, I like the SX's for their thick sidewalls and all around burly design, they can take more sidewall abuse before ripping than the irocs from what I have seen.
Remember Irocs are directional so your spare will be one side or run backwards when needed on the wrong side, if that even matters to you. LOL. I've also "heard" that the Irocs softer tread wears faster than the sx's. This is hearsay tho, not experience.
Those inner air locks are a good $500+ a set of 4 from what I have read online, that's getting close to half the price advertised. this is the better deal imo. Offer the guy $1000 cash in hand, see what he says.
http://www.innerairlock.com/beadlocks/
I'd also offer the guy with the SX's $600 cash in hand, people like cash in hand, see what he says.
Then if they both said yes, I would grab both sets, swap the sx's over to inner air locks and sell the 5 Iroks and the extra wheels I had left over for $800 for the tires obo and a couple hun for the wheels obo, lol.
It is your choice in the end, this is just my 2 cents, lol.