At this point i believe your only path is a long demo ride. Barring that you aren't going to be able to decide. chances are it won't be AS comfortable as the BMW. But it is comfortable. I find it extremely so, but that doesn't tell you anything. You need to find a place that will allow you a couple hours or even 1/2 day ride. Offer to pay if necassary. but understand there are people who don't find it extremely comfortable and thats the only way to find out if you'll be one of those or one like myself. I will say i believe you will find it much more comfortable than the bonnie, and i have read no less than 4 or 5 people who have said they feel it handles better than their bonnies, one guy even saying better than his thruxton. I had a bonnie variant, a speedmaster. And i can say w/o a doubt it's a far superior bike in the comfort and handling categories. If you love the bonnnie i think you will be over the moon for the Tbird. Moving to that from a speedy for me felt like going from a toyota to a mercedes.
But alas, as i said before you need a long demo because everything i or anyone else tells you are just the opinions of others.