Join Date: Jan 2011
Location: Colorado Springs, CO
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Out of the ones you posted, I'd go with the GT, simply because it has more gear options.
Entry level bikes are meant for trail riding and maybe small jumps of under 2'. Don't get too crazy because those bikes are not meant to handle jumping or extreme downhill riding.
Things you should look for:
Rockshox forks are almost always better than anything by Suntour. Dart 1 pretty much suck though. The Dart 2 is better. Then 3. Rockshox Toras are a step up from that. They are number also. The higher the number the better.
The more rings on the cassette, the better. So 9 speed is better than 8, etc. It just gives you more gear options to get up and down the hills.
As for derailers and shifters, most are made by Shimano or Sram. Shimano from decent to best goes Deore, SLX, then XT. Sram counts up. So x4, x5, x7, x9, x0. In your budget, you will find the lower stuff.
If you haven't checked out mtbr.com, you should. It is a mountain biking site with tons of knowledgable people.