Just my 2 cents as I have both the SM and the older BMW R1100RT.
For long distance touring at higher speeds the BMW cannot be matched. In the very tight twisties it is a bit of a brute and can be somewhat tiring.
I have been up and down Europe (right through to the Middle East) on it (both one and two-up) and it has always performed beyond expectatons.
I would suspect that the new R1200RT would be an improvement over mine given the lighter weight and the super advanced brake and suspension systems.
This last July I tested the SM's merits and was quite amazed. The first test (one-up) was my run up to Asendorf in Germany for the European BA.COM rally (see write-up in the "Rallies" section). After crossing on a ferry from Greece to Italy, I did a straight 29 hour run from Brindisi in the south of Italy with a 2-3 hour break just north of Innsbruck, Austria before continuing on all the way up to Asendorf (between Hannover and Bremen). A total of approx. 2.300 KM'S (including a small detour to Venice).
Picture below shows the handlebars used (similar but narrower than the America's and the "Bikerfriend" bag which played an immense role in supporting my back).
Two weeks following the Asendorf run I headed out (two-up this time) and spent 2 weeks or so travelling around the crappy and, for the most part, extremely tight twisty secondary roads (I avoided major highways) in the north and west of Greece to include four island hops (write-up coming soon).
Having done similar trips within Greece on the BMW before, I can safely testify that the Triumph is both more suitable and enjoyable on this type of terrain (not that I ever complained on the BMW).
So, to summarise:
Long distances, moderate twisties, more luggage space, 100+ MPH (on major roads/highways): BMW RT.
Long distances, moderate and/or very tight twisties, less luggage space, 70-75 MPH (on major roads/highways): Triumph SM.
In my book, given the abovementioned parameters, the bikes ultimately have similar comfort levels (excluding the BMW's built-in wind protection and heated-grips) and the only other advantage the BMW has is the total distance travelled within a specific time frame because of it's obvious higher cruising speed.
