Application here is a tachometer, I'm thinking I'll have 8 magnets on the perimeter of a drum, likely spinning no faster than 120 RPM, so signal will be no greater than 16hz. Just off the top of my head, given that some reed switches I've looked have open/close times in the millisecond range, that should be fast enough, even with debouncing taken into account. But I think that with, e.g., a lifetime of 20,000,000 switch cycles, that's 347 hours of operation (if I figured correctly) -- well, probably plenty for me, but if my idea for solid rocket fuel takes off, maybe not. So, I'm thinking Hall sensor. But other than the very basics, I don't know quite how to select one.
For example: Optek OHN3120U looks fine to me, but looking at the "425G Trip, 50G Release", I assume those are gauss figures. If I use the Neodymium Diametric Disc N35 6x2.5mm down at the bottom right of that page, it seems they're plenty strong to activate the sensor. But with 8 of them around the perimeter of a 5" diameter drum, will the flux density at the midpoint between 2 of them be low enough for it to turn off? The drum will be PVC. At the other end of the magnet spectrum, cutting bars or dots from magnetic vinyl stick-ons comes to mind, but there are no gauss ratings for those.