I have a Longacre Digital unit that has a rim clamp.

If you have rims and steel hub that can accept a magnetic bubble gauge, I'd buy that.

My Digital unit has button click procedure that I always forget and have to re-learn. Then I sometimes don't think it's calculating it right.

An old fashioned mechanical bubble type is more fool proof. For caster, you rotate the gauge to level, then turn the wheel the angle required per the tool. The required angle sweep is usually written on tool or you can write the angle on blue tape to remind you.

The turn plates make the job easier for me than having a digital gauge vs bubble. The turnplates have the angles on them so you can get your caster sweep real accurate. And with plates you don't have to roll your car back and forth every time you make an adjustment.

I got turnplates used from a circle track guy since this picture. Much quicker and easier now.