The yoyodyne is inexspensive ( comparatively speaking) and not very complicated piece. easy to install
The stm is a nicer piece, a little more complicated to install. a little more expensive.
The both function similarly by ramping up ( slipping) using steel balls that go up a ramp to disengage.
Both seem to work well with no complaints.
You can call me at the shop if you want some pricing
(309) 526-3246