Being this is a Behringer thread, here is an article that shows the differences between 15, 35, and 55 Behringer Eurorack systems. Moog versions were of course not Eurorack.
This isn’t gear in progress though, these are out and available. The 2500 modules are available too, as well as Series 100 ( Roland ) modules. Behringer makes a ton of inexpensive and good Eurorack.
BTW : There is a thread for the Behringer 15, 35, 55.