I'm pretty biased to Shimano. Granted, I haven't used much SRAM gear, but Shimano just has never let me down. Even the mid-range offerings for Shimano are good. Then once you hit SLX/XT and sometimes up to XTR, the performance differences tend to be marginal, while the price difference is not.
I'm assuming the Guide came stock on the bike? Shimano just released a line update. Jenson has the previous Front/Rear SLX for $80 each right now ($160 for the set),while the XT is $98 each. I just couldn't pass up that price on the SLX! I also picked up a 2-piece 203mm rotor for $30, just couldn't pass the price.
Now, most people would tend to think that 203/180 rotors would be overkill for my area and for this bike. But, 27.5" x 3" tires with a 240lb rider, feels dang good to me.
Yeah, I'm with you, street riding scares the crap out of me! With the terrible Austin traffic and drivers, and half of them stoned, I'll take the trails any day.