I wasn't aware that the 145b could run 6, I thought 7.0.1 was the minimum on it. (I know the original lineup of 100, 140, and 170 can be forced to run 6.0.8L, but I didn't think any of the later models could even be forced into it.)

I find that it all depends on RAM. With less than 8 MB, a 7.0 or 7.1 is definitely more snappy than 7.5. With exactly 8 MB, 7.5 or 7.6 become possible. With more than that, 7.5 or 7.6 is very usable. (And "more than that" includes the use of RAM Doubler, but not built-into-Mac-OS virtual memory.)