I am in the market for one of these bikes; i like both brands and am coming in with a open mind. Fortunately, one of our local shops, Pacific, sells both brands as well as Yamaha. I went for a back-to-back test ride ride, going BMW, Triumph then BMW again. my wife did some pillion too... My preference was suprisingly clear.
For engine/gear box, I felt they were tied; even though they are profoundly different. Depends what you are looking for - the Triumph is the hotrod but i like the torque characteristics of the Bimmer. I am coming from a FJR and it kicks the crap outta either; my top priority is not speed.
Suprisingly, for everything else it was no contest ... All BMW! it felt more nimble and just ... Better. Add the aftermarket advantage for BMW and, for me, it has been decided.
Two footnotes ... No comment re seats as I would chuck both
... And no I am not a BMW owner!