Like most of us who have been shooting for decades - I have a large collection of meters - and some still work!
Everything from Gossen SBC, Sekonic 308's, Gossen digital, Digisix and more. The one that I use for checking my guess work tends to be Digisix - attached to my vest, and always along.
The Gossen SBC's tend to be used for set-ups or more precise metering - but it is almost 1/2 the size of a M!!!!!
You will probably find that if you stick to one film or film speed - the meter gets used less and less. Mainly to check a guess or for tricky light situations.
The Leica MR meters are convinient, but require Mercury batteries and they tend to scratch up the top of the camera. Nice 18 degree angle though - perfect for shooting with a 90mm lens.
The VC meters are good - again, I use them as handheld meters - rather than leave them on the cameras.
Unless you are shooting narrow latitude slidefilms - metering is not that critical. Color Neg films have a huge latitude as have black/white medium speed film. It is easy to get caught up in metering madness to the detriment of taking pictures.
The M's are compact cameras - why add something that is almost the size of the camera just to get the exposure?