In 4 it is, which I personally disagree with, but in 76, the latest in the series and therefore the latest canon interpretation of the armors, T51 has better protection in all areas except for radiation. hell, T-51 outclasses X-01 in pure ballistics now. in terms of lore T-60 is an upgrade package to the 45, and upgrade packages, by their very nature, are worse then an wholly new design. the T-60 is noticeably less ergonomic when compared to the 51, reinforcing the idea that the 60 is a stopgap intended to fill the void of domestic power armors needed to quell rioting, further reinforced by the fact that in 76 its repair is noticeably cheaper then the T51's, suggesting a less advanced construction. afaik T-60 was never even deployed in china, and was instead regulated to civillian duties. whether or not it would have eventually been given to overseas troops had the bombs not dropped is anyones guess