I have owned and used them all so here are a few of my thoughts. I believe there are goods and bads with all of them though.
Foley - Slow, easy to learn but not a fan of measuring off the center of the reel shaft. Not the most accurate way of grinding a reel. The bedknife grinder is as simple as it gets to use just not the most accurate.
Bernhard - Quick, easy to learn and teach. The reel grinder is good they do get some hickups on occassion depending on the model with the electronics (I had the 5000). Not a fan of the bedknife grinder.
SIP - Quick, takes some time to learn and understand the process but certainly the most accurate. Bedknife grinder "imo" is bar none the best out there because of how it grinds the bedknife from the pivot bolts so the distance from pivot bolt to front edge is the same on both sides.
Here is my explination on the SIP bedknife grinder a few years ago:
https://youtu.be/cGo2jm11s4IObviously this is my opinion I always recommend demoing them all to make your own judgement on what you feel you can get the best results with.