The HB-125 uses the oil in the primary with a plunger where the M6 is just spring loaded.
Cannot speak to the reliability of the 125. but it seems to be well liked. The M6 has been around a long time, is very simple, and is proven over time.
I got mine for $68.95 from Jireh Cycles. Their site is kinda clunky, so it's better to call.