Good advice so far.

X-Mart bikes are really disposable bikes and not intended to be actaully ridden off-road. If you need something to go back and forth to school then fine get one, but if you intend to actually ride one on some trails it'll break down soon.

Can you ride one on trails? Sure, but it'll be heavy and give you a ton of trouble maintenance wise.

The Specialized mentioned is a good bike, there are a lot of others as well. Go visit your LBS and test ride as many as you can. The $300 price point is highly competitive and you will see a lot of the same components on each different manufacturer's bikes.

It'll all boil down to, fit and style. Each manufacturer has their own specific geometries to what they think is best. What is best for you, is not best for me. Test ride as many different bikes as you can. Find the best 3 that are the most comfortable and then buy the one with the best paint job!

L8R