[KTL]

Measure the old piston diameter and the new piston diameter. If they are the same, don't worry about the hollow vs. non-hollow. Note that you need to extend the piston out a bit to ensure you're measuring the correct part of the piston- the clean area that seals the fluid in the caliper.

The hollow pistons are stepped on the end to prevent pad noise. This is the step that you have to set at the 20 degree angle. The hollow pistons also have less thermal mass to reduce heat transfer into the caliper. So in theory the hollow pistons are better.