[rafael_macia]

I have always used diopters, and have never understood how people with glasses could see frame lines without them. Leica used to make up to -5, (up to what plus I am not sure of), but now they, (Leica) only produce up to minus or plus 3. Aftermarket, produces the same range, for less cost.
Roughly, which diopter is best can be judged from your glasses prescription. I am a minus 4 because 20/400 is my Rx for glasses.
different cameras have different built in diopters of mild correction. which should not affect your choice of which diopter to get.
At one time Leica (Leitz at the time) offered a diopter correction "wheel", some dealers had where you could look through various strength diopters, to help decide which was best.
I would recommend you try different diopters at a Leica dealer.