As a (very) general rule, the mountain bikes in the $500 price range are better built than other bikes. The designers assume they will be abused, so the frame, fork, and components are built to take abuse.

There is not a big difference in quality between any of the major brands. I'd pick the bike by the quality of the shop. The best shops get the bikes built right, tuned right, and take care of their customers when the bike needs adjustments, wheel truing, and tune-ups.

And, the closer the shop is to your home, the easy it is to ride by for adjustments...the shop six blocks from home is more helpful than the "discount" shop six miles from home.