I think the big difference between him and the RSA horses is that it is not unusual for the latter to improve a stone and more in their second season. It's hard to imagine TB having that kind of scope at his age. Bobs Worth probably improved past TB today. By the time Cheltenham comes round, it isn't unreasonable to imagine his having improved another 7lbs in the interim.
Yep, its why one is 5/1 and the other is 25/1 ante post. If it was softer or worse, I'd allow him to take his chance, and wouldnt categorically rule him out.