I've got a Thruxton and it set's in garage most of the time. I won't sell it. I really love the bike's styling. I've always been a sport and touring bike fan. Didn't really care for cruisers but I can tell you from my point of view, I really love riding and the looks of the T-Bird. I'm relaxed and actually enjoy riding more, even in town. I pretty much smile all the time while I'm on it. Don't really know why, just do. Maybe it's like those old pair of jeans that fit just right and are really comfortable. Either way, I now have two motorcycles and I'm not willing to sell either.
In your case, I suppose it's the one you like the best. If you ride the T-Bird first, then you'll be disappointed in the Bonnie. Both are to close in purpose. One is not radically different for the other. Ergonomics are similar with the exception of the forward controls and neither are considered high end performers. Easy of maintenance is similar in many respects, with the T-Bird having the advantage. There 's actually less to deal with on a regular basis. Two up riding, easily favors the T-Bird as well as hauling luggage and gear on trips. Comfort I have to say also goes the the T-Bird.
I don't think either is a wrong choice but for all-round performance, you're not going to beat the T-bird. It cruises effortlessly at higher speeds on the highway where the Bonneville struggles. It handles extremely well and is light on the steering. You never feel uncomfortable.
So it all comes down to your personal choice and perceived use.