I have had the cheap Amazon rails (including Velbon x/y versions). The problem with most is the wobble. Second is the lack of Arca style QR. While generally a RRS fan, their macro rail gets less than stellar reviews, so bought a Hejnar and quite happy with it. It is the MS5P-8, works well at 1:1, can do some simple stacking. With MPE 65 it is a bit trickier, but still works ok. One turn of knob = 1.06 mm stage travel, so if your DOF is about 250 microns (1/4 turn) that can be done manually. At 1:1 at f/2.8 you are about at limit. at 5:1 and f/2.8 DOF is around 0.04 mm, so a bit tricky to do that consistently with a helical thread.
Re vernier callipers, just depends on the magnification you need. If you go much above 5:1, then certainly vernier. Or Stack-Shot. >>5:1 without stacking, I don't see the point. Stacking is so much easier with Stackshot. Just make sure to get the X3 controller even if you only use single axis.
Re bellows, had them, and recently got one back for UV reflectance z-stacking with an M39 enlarger sense (Nikon EL 80/5.6 silver). The problem is the additional bayonets and their intrinsic play. Camera-lens: one junction. Camera-bellows-lens: two junctions. Then there is also some play in the front and rear standards. They have certain utilities (e.g. T/S options with some, WA lens reversal, mounting 160 mm tube length microscope lenses with RMS adapter, ...), but a blanket "bellows is better for macro" is nonsense. There are also distinct problems, such as handholding a below set-up [good luck!], fragility of the system, light leaks, ground clearance, to name just a few. Only get a bellows if you really need it.
