Trying to choose between the two bikes, I would trade the Bonnie in and keep the BMW. I think the Bonnie and T-Bird are too similar in nature to have both,keeping in mind your two-bike thinking. Having the BMW and the T-Bird would make for a more useful and enjoyable pairing.
Actually, now that I have the Tbird and enjoy it so much, I probably will sell the Bonnie. I really love the Bonnie but as you said, the two are very similar in nature. I just happen to like the nature of the Triumphs more than I liked the nature of the BMW. Not that the BMW isn't a great bike, it's just not the kind of bike that I enjoy for the kind of riding I do (2-3 hour cruising).
So if anybody out there wants a 2011 Cream/Brown T100, let me know.