The electronic ones need a material that's conductive; aluminum, steel, brass, copper, etc. (won't work on plastic or wood). The ones I have are metric with a 20 mm shaft, 10 mm ball. I bought a couple of 20 mm R8 collets that are dedicated to the electronic finders. I have DRO's on both mills and usually work in English units, but it's easy to switch to metric if using the electronic ones (or try to remember that 5 mm is 0.19685").
I use a mechanical edge finder 99% of the time. Not because I'm deathly afraid of spindle runout (if it's greater than 0.002" I've got other issues), it's what I'm used to. My mill work is done on a Bridgeport with an ER collet chuck. Most of my work involves using either a 3/8" or 1/2" collet. Out of laziness, I have edge finders in both diameters to save me from changing ER collets.
I use an electronic probe most of the time on my Tormach CNC mill, but also have a 3D taster (Haimer). The Tormach's original owner had an electronic edge finder, but I've never used it.
Bruce