At that price range the components will all be about the same. One may have a better rear derailleur or a better fork, but really it all boils down to frame geometry, paint color and graphics.
Test ride each and buy the one that's most comfortable.
Personally, I'd stay away from disc brakes at that price range. Unless you get Avid Mechanical discs, you're wasting your money. Buy a "disc ready" bike and upgrade down the road. Spend more money on a bike with the better fork.
My cost assesment priorities are as follows:
Best frame for the money.
Best suspension for the money.
Best cranks.
Best brakes.
Best Shifters.
Everything else.
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger