You may be unaware of Gonzo (as I write this, your post count shows 21, so you may not have been on this forum long), but as you can tell from the above dialogue, he has strong opinions that he will express in whatever manner he chooses, whether it may be found offensive or not. This is the most tactful way I can state this.
Anyway, on to your question. The Specialized, Gary Fisher, and Trek models you mentioned will all be solid bikes. Typically you start getting pretty nice bikes at $700-800 and up, so with your price range of $1000-1500, you can get a very nice bike. Based on your questions, I assume you don't have a long bicycling history. With that assumption, I'd recommend making the rounds on your local bike shops to check out what they offer, as they can help you more than an internet mailorder site. (SuperGo does have great prices, but they won't be able to help fit you to the bike like a local shop, and you'll have to do all the assembly yourself. Unless you're fairly experienced with bike work, you'll like the results from your local bike shop better. Also, don't mailorder a bike and bring it to your LBS for assembly -- they don't like that much, as you can imagine.)
For hardtail vs. full-suspension, this has come down to largely a matter of preference. I've had FS, sold it, now only HT. My choice. Generally, though, at any given price point you will get a lighter bike with better parts on a HT than on FS. However, if you're shopping the $1500 point, the differences may be minimal as far as components.
For brands: Generally, you will get "more bike for the buck" from a major brand: Specialized, Trek (including Gary Fisher, and all the other brands Trek bought out), Giant, etc. These companies have the buying clout to underprice the smaller competitors. You often get a lot of house brand parts on these bikes; while they typically work fine, they tend not to be the lightest.
For steel vs. aluminum vs. other materials (carbon, titanium, mixes): a majority of what you'll find is aluminum. There is way too much to go into here as for the benefits of one vs. another. Generally, however:
Pro: cheap to buy, stiff (efficient), doesn't rust (don't worry about scratches), easily available from all the major bike brands, light. Typically the material used for FS frames due to its stiffness.
Con: stiffness translates into punishing ride (this is the tradeoff for efficiency) on hardtails, can dent easily compared to other materials, more difficult to repair (failures tend to happen suddenly and catastrophically).
Overall: a good choice for many riders for a price/ performance standpoint.
Pro: nice riding "feel", easily repairable should you break the frame
Con: rusts (you can treat the tubes inside to address this, but it's not the easiest thing to do), limited availability (primarily available from smaller brands and custom shops), typically slightly heavier than alu frames (though you can get very light steel frames)
Overall: steel remains the choice for many HT aficionados due to the "feel of steel."
Other materials: -- carbon, titanium, exotic mixes:
Pro: Can be made to ride nicer than alu or steel
Con: Expensive, difficult to repair
Overall: too many options to generalize. At $1000-1500 you'll have a few options here.
You've got a lot of choice to make. Best recommendation is to go around to your local shops, talk to the staff, tell them where & how you intend to ride (and how much you want to spend), and go with their recommendations. 1-year-old leftover bikes can be a great way to save a few $ and get basically the same bike as current models.