The RKSS is an xc race tire and the RQ is an AM tire. Plus, it looks like only the 2.4 sizes of RQ have black chili - and those weigh 380 grams more than the 2.2 RK SS.
I have the RKSS 2.2 tires and they are pretty thin. That's ok for race day, but I wouldn't want to run them in really rocky environments where there's lots of stress on the sidewall. The RQ is a totally different deal though, I haven't tried them but an 850g AM tire is going to be way way more durable than the RQ.
For trail riding, how about the WTB mutano (new tire for '10) in 2.4 and a nobby nic 2.4 snakeskin for the front? Both are 650-700 grams or so, both have extra sidewall protection, both are fairly large tires. I like the old mutano raptor 2.4 tires, I've had a few sets of those. As far as I can tell the mutano is an updated version of the mutano raptor, with bigger looking knobs and sidewall protection.
Both the mutano raptor and nobby nic are a breeze to set up tubeless with stans - I'll bet the new mutano is just as easy.
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