Thought I'd update this thread seeing as it's 6 years old now.
I had a 2015 Tiger 800XCx. At the time (i.e compared to F800GS from 2015) the Tiger was a far superior offering, at least in XCx spec, and if we're talking asphalt. For offroading, I'd choose the GS.
For a start the standard equipment was better; RBW throttle, the engine was FAR better, the suspension (fully adjustable front and back, though no electronic option) was KTMs WP and the star of this bike after the engine, heated grips, cruise control, 4 riding modes including one customisable Rider mode, engine bars, skid pan, 94hp, centre stand, great finish, much more comfortable than the GS and that tripple.... have I mentioned that?
I rode two F800GS's for two days solid (hired in SA and LAX) and I liked them but they were no way as good as the Tiger XCx.
Now, I only ever tried one other Tiger as a courtesy machine, an XR (not t) and I have to say that I noticed that the lack of good suspension ( together with brakes - in those days - was a weakness of Triumph) made all the difference. I believe the XRt has Showa or something of equal quality and therefore also good.
The ONLY things I didn't like about my Tiger were the 21" front wheel (because with that suspension it can corner with the best of the them, but you're always wondering if that front will wash out on you) but the GS also had that, and the topheavyness of it. You get used to it, cornering takes a little more thought; you have to be a little more precise and think a little further ahead, perhaps but it is a pig to pick up if you drop it, and you will drop it because the angle of no return is not very wide. .......ask me how I know!
Sadly, I can't offer any comparison with both new models but I think that I'd still go for that tripple in XRt guise or Adventure today because once you have tried that lovely tripple engine, it's a big draw over the equivalent twin.
The comparison isn't the same between the 1200 and the GS, though. What swayed me in the end was heritage and that all important weight distribution. I could just about cope with the 800 being top heavy, a 1200 would have been too much for me, and ultimately, it's what really did it for the Tiger 1200 for me.