As far as 'lasts longer' once you're into the $500-$800 price bracket, it'll be pretty much a wash as far as durability is concerned. These are entry-level 'decent' bikes. They're not throwaway 'cheap' bikes and not high-performance either. Kind of the 'Toyota Camry' of bicycles, competent, dependable, affordable, but jut not super-exciting.

The Avalanche has some nice upgrades, like the hydraulic brakes, but maintenance on them takes some knowledge (and sometimes tools) that's not in the scope of your typical home bike mechanic. The 9-speed drivetrain is a good step up, too; but if you're on a tight budget, is it worth the extra? Nothing wrong with 8-sp. Heck, i've got some vintage bikes running 7- and 6-sp and I don't find them wanting for the kind of riding I use them for.

Finally, on Suspension Forks. BikeForums is very anti-suspension, and takes a dim view of MTB in general. There's just a lot of graybeards here who think that bicycle design peaked in the early 80's with the Miyata 812. Inexpensive suspension forks do add weight, it's true, but light weight air-sprung forks take a lot more regular maintenance.
A simple suspension, set up right can be just fine. A 'sport' grade fork, like the SRs on the GT's you're looking at should have a preload adjustment. Setting the preload higher will result in less of the 'bobbing' that people grouse about. You want the fork for fending off big bumps, and use those 2.1" tires for the 'road noise'