I'm not really new here, but I'm with my F800GS [3 Aug 2010], it's a 30 Year Jub. version, with BOS exhaust, NGK iridium plugs and UNI foam airfilter fitted, etc.
Must say that I can't compare with the Triumph Tiger 800XC, because I haven't driven one. BUT the 800XC looks very good.
But .... I'm surely not going to change my F800GS, besides of the lack of troubles I have [NOT] with this lovely machine, it's MILEAGE is unbeatable !!! ;-).
On daily home/work traffic [with some in-town traffic], I easily make 1:25 km [even 1:26-27], which is at least a mileage of 58.79 [US] or almost 71 [UK].
I think the 800XC is doing about 1:18 km [mileage 50-55 UK].
Of course there are some problems with the F800GS, I drove a F800ST for 4 years with lots of [beginners] problems.
But as I allready wrote, now with 16K [miles] on the GS with no problems at all, one can say the F800GS is a good bike.
I red in the UK Triump 800XC forum, there are some 800XC's having stalling problems, this is not the case anymore with the F800GS.
So, if I look to the fuel consumption, I think the F800GS will be cheaper at the end than the 800XC ;-).
If I should drive 20K a year [it's even more], it would be a difference of at least 300 ltr - The Netherlands is quite expensive for fuel, today 1,63 euro/lt. So > 300 ltr would be almost 500 euro a year.
F800GS, I can't hardly think about something to improve the bike.
Some complain about the seat comfort problem
, the saddle of my racingbicycle, that's a real comfort problem