First, you can tour on ANYTHING.
That said, the Sportster really doesn't have the range. You'll be stopping for gas a LOT more. Wind/rain protection is nil unless you spend a fair bit on it (altering the basic looks of the bike too). Handling isn't exactly amazing either. Belt drive is nice and the dealer network is good though. I rode 1000 miles with a bunch of Sportsters last year (check out the XL-forums on the web for tons of info on these bikes) and most guys were stopping every 100 miles to stretch or get gas. Luggage options are almost all soft bags for this bike too, so you won't get truly waterproof luggage and storage will be less. I'd say comfort is lower here too than on all of your other options. If you like the image of the cruiser, this is your winner.
The BMW 800GS is a whole different bike. If you plan to go off-road, this is clearly the best choice from your list (for highway riding it's fine too, although I'd favor the 919 or FZ6). You could buy TWO outfitted FZ6s for the same price though. ABS is a good option and there are lots of ways to prep this bike for touring (none will be cheap - have your looked at the cost of Touratech parts?!) Dealer network is less extensive, costs of maintenance are higher too. This bike is NEW, which means that there are some minor bugs (ignition cutting out / stalling is known; immobilizer can leave you stranded; some minor mapping issues perhaps). Range is better (about 200 miles - small tank with great fuel economy).
Hondas are good bikes. I don't think the 919 is quite as all-around capable as the BMW/FZ6 stock. I've sat on them and read a bit, but haven't ridden one.
Aprilia? Don't know enough to comment. Dealer support and aftermarket support is gonna be an issue here.
FZ6. Obviously I'm a bit biased, since I own one. As others have said, the FZ6 is an excellent all-around bike (unless you plan major off-road trips - it just isn't designed for that although the upright seating position makes it possible to stand on the pegs for short trips down dirt roads). It's comfortable, has good range (safely 200 miles) and there are TONS of aftermarket luggage options available. I disagree with the previous poster about the stock seat - I think it's the BEST on any bike I've ever owned. I did 1067 miles in one day a couple of months ago and the stock seat was great for me. I've done a couple of 500-mile days as well with no problems. The FZ6 really shines on paved twisties, but it's also a good bike on the high-way too. I love the seating position on the FZ6 and it's very reliable. Maintanence intervals are long too (longer than all of the others you listed except for the Sportster). For your $$ I think this is the best value. You even get a centerstand as stock.
You might want to consider a V-Strom 650 as well. Good all-around bike that's more capable on dirt roads than anything you listed (except the 800GS of course, but the V-strom is much cheaper than the BMW to fix when you drop it). The Versys is also a very capable bike if you're tall enough for it. The Buell Ulysses is also an interesting animal as well. You'd get the HD belt drive + a sporty ride. On paper the Ulysses is excellent, although in reality the reviews vary (I found it fun to ride, but they are known to have a variety of maintenance issues - especially with stalling, sputtering, backfiring; the cooling 'fan' is also a beef with owners....if you ride one you'll either hate it or buy one).
The FZ6 is also different since it's an I-4 engine. It'll have less torque than the other bikes but similar or more HP. When I was looking around, I decided I wanted to try an I-4 over another V-twin or Parallel twin, so that helped tip the balance for me.
Your leading 3 choices are really quite different. FZ6 is much cheaper than the other two and falls somewhere in between in terms of style and substance.
but was otherwise fine. 
