I don't know about the frames on the TZ series tractors, but several folks on TBN have TZ tractors with loaders and seem very happy. There is also a Case/Farmall equivalent of the TZ sub-compacts and a few folks here own those and are happy with them. Most seem to really like the loaders. As they have been on the market for about a year, it is hard to tell if they will be still working hard 15 or 20 years from now.
As for the BX series and their loaders, they are well proven and have been on the market for several years, in fact they started the sub-CUT size tractor trend. I don't think you will find a more refined small tractor on the market. Of the folks here on TBN, there are dozens, perhaps many more than that who are happy BX owners.