I am not sure why CS-Lab say that but from my point of view the A is the controller of choice. I have used the S and M on two other retrofits and both are excellent controllers, the A however has advantages over the S and M and will be what I use on the big lathe when threading can be done correctly (Mach issue at the moment but Brian is working on it
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With the Step/Dir models there is no encoder feedback to update Machs DROs, that means any time you take Mach out of reset you need to home as there is a good chance the axes have moved slightly from previous positions, that is especially true of a Z axis. When I press Reset on my Chiron the Z will fall slightly as it takes a few mS for the brake to hold, the X and Y may also move depending on forces acting on them. As the A updates Machs DROs from encoder values Mach always knows exactly where the axis is so when I take out of reset I do not have to re-home. Also moving an axis manually when servos are disabled will update Machs position.
Another nice thing the A has the advantage over the S for is the Index homing. The S can do it but depending on servo drives you may require additional hardware, the A you dont need anything extra.
The only disadvantage the A has compared to the S is less I/O but an extra I/O module would make up for that, obviously thats extra cost compared to the S.
So basically with the exception of the amount of I/O the A does everything the S does but it also does more which in my opinion makes it the better of the two.
As said however the S is an excellent controller and you will be delighted with it.
Hood