The Thruxton is very understressed (therefore quite reliable), has 3 cylinders, is British, is quite characterful and dirt cheap compared to the GS. Anyone done miles on the Triumph?
I've ridden the original Hinckley Bonnie and the later T100. The T100 is a really nice machine to ride, when there's a few more around second hand I'll be having one. The Thruxton has more go but is compromised, for longer journeys, by the riding position.
A Thruxton to Finland? Tell me you're not serious.
I don't doubt any bike could do it if in good condition (ie new or well prepared) but I don't think you will enjoy the riding half as much as you will on a GS, especially if you get the chance to venture a little off the beaten track, which the GS specialises in, and the Thruxton definitely does not.
Just because Ted Simon endured a Triumph for his 4 year round the world trip (Jupiters Travels) don't feel you have to. If Boorman and MacGregor took more modern GS's (Long Way Round) you know they make good sense.
Add in heated grips, ABS, quality hard luggage, even GPS if your budget allows (luckily for me it does) and the GS wins it for me anyday. Throw in a cast-iron warranty with worldwide cover and its no contest. I've had so much fun touring on my GS on roads that would kill my sportsbike.