I had a 2007 RT and a 2003 LT and loved them both. The only problem was that my inseam is just a bit short for them so when stopping on uneven ground and maneuvering out of tight places, I've come close to dropping them.
I now have a 2010 Thunderbird that fits me ergonomically much better. It is a very cool bike. I love the sound, it handles very nicely and it is just plain fun to ride.
Differences: On the RT, I felt like I was going 60 when I was going 90. After the Beemers, the 'Bird felt like it was going 90 when I was going 60. I think this is due to more sensory input - more wind, more sound, more vibration. The vibration isn't bad, but it is certainly more than the RT or LT. The increased wind isn't a bad thing (at least not in the summer) it's just different. I may feel differently about it when winter gets here.
The T'bird is a blast to ride, but so is the RT - just different. I recommend you spend at least a half hour at freeway speed during a demo ride to assess the differences yourself.
I tend to agree with one of the others that you would be better off trading the Bonnie for the Bird instead of the RT. That way you have two very different, but equally enjoyable motorcycles depending on your mood and the type or riding you are doing that day. Could I ride the Thunderbird across the country in variable weather and enjoy it? Absolutely. Would I be more comfortable doing on an RT? Absolutely. But both would be an incredible amount of fun.