[Flyinfool]

The 501 is a heading hold gyro.
I am still using one on one of my old helies.
I think it was the first HH gyro that Futaba made.
The 401 is noticeably better at controlling the tail in fast backwards maneuvers.
The 401 is a LOT smaller and lighter in weight than a 501.
The 501 was about the same price as a 401 but the 501 has not been made for many years, so if you are seeing it on fleabay for the same as a 401, then it is way over priced for old technology.

Now if you had a typo and were referring to a used 601 at near the same price as a 401, then this a whole different discussion.