I've been running hydraulic disc on my bikes since 1997 so disc brakes have been out for awhile. If you ever plan to ride a trail that has damp areas, creek crossings or even get caught in the rain you'll want the one with disc. For mechanical disc like the ones shown in your link, run full cable housing to the brakes if it doesn't already have them, you'll get better feel & more consistent braking.
If you like riding and have decent skills you'll quickly outgrow either of those bikes so get prepared to upgrade components or get a new bike. As was mentioned earlier mtn biking isn't a cheap hobby and when upgraditus hits you're going to drop a bunch of money quick on even mid level components. People think shooting is an expensive hobby but when you start upgrading frames, forks, wheels, shifters, etc it's amazing how fast the dollars add up. I'll post a pic of some of the components on my last build that cost as much as the last ed brown I bought.
Here are a few other tips for you.
Try on a lot of helmets and get one that fits even if it cost more. Nothing will ruin your ride faster than a helmet that pinches or is constantly bouncing around on your head.
Get a bike specific mutli tool to take with you on rides and learn how to work on your bike, chains will break, tires go flat, bolts come loose, derailleur hangers bend and other stuff is going to happen when you're riding. Crank brothers and topeak both make good multi tools that will handle most mechanicals.
Take a tube patch kit or spare tube and at least a spare master link with you. Practice changing or patching the tube and repairing the chain before you actually have to on the trail. Walking a bike 5 miles back to your car sucks when you have a flat or broken chain you can't repair. If your local bike shop won't let you have some old stuff to practice on pm me and I'll send you a tube and old chain.
Take plenty of water and drink while you're riding to keep from getting dehydrated, also take some type of food or bar like a clif bar or gu shots.
Take a banana to eat after your ride to help prevent cramping. It's hard to drive home when your legs lock up.
Take clean clothes or at a minimum shorts to change into after your ride. You don't want to stay in your cycle shorts very long after a ride or you may develop crotch rot.
Get a basic first aid kit and supplement it with big bandages & tape to keep in your car. If you auger in unexpectedly, the strawberry or dirt rash will probably cover a fairly large area and typical band aids won't cover it or stick to your sweaty skin.
As much as I try to use local bike shops they generally aren't competitive once you move into higher level components. I've bought stuff from Shane at bear creek when I'm up that way just because he's also into shooting and has a cool shop. I learned how to build my own bikes including rebuilding forks and building wheels a long time ago so I don't need a LBS which is great since there isn't one in Newnan anyway. Besides that, none of the bike shops with a 20 mile radius of me can compete with Jensonusa or Pricepoint, the two big online retailers.
Be safe and have fun.
