moving coil vs. moving magnet cart. for Rega P2/3

Out of three you've mentioned Rega ones are slightly better than Sumiko that seems to my taste too "beefy".
Benz Glider will probably cost more than whole TT meaning you won't be able to taste what this cartridge can do. BTW Glider to my taste is too soft.
The best choice among MC IMO is Goldring Eroica that you can catch brand-new for a-bit above $200 if you search.

If you look over in the "Lenco" thread you will find some info on this.A Rega arm has an effective mass of 11gms and this is considered medium mass.A MM cartridge really works best on a light arm and an MC is more suited to medium mass arms like the Rega(generally speaking).The Sumiko is a great choice for the Rega and is also around 10Hz cartridge resonance which is near perfect.

I used a Benz Micro Glider L2 with a Planar 3 for a couple of years before I traded up to a VPI set up. It was my first taste of what a moving coil could do that a MM cart could not. After that there was no looking kcab ;-). Go with the MC but make sure your phono pre can hand your preamp the goods.