I revisited my FM3a with super lightweight-superlightvalue 50 f1.8 AiS (60 EUR, mint) last week and shot a handful color neg film.
It handles subjectively lighter than a comparable M (weight wise).
It is not exactly bigger, but just differently shaped with the prism housing.
The 50 f1.9 is not really bigger than a 50 Cron (it is lighter due to some plastic and less dense glass but veery smooth and well made - doesn't feel "cheap" except the plastic aperture ring).
It's wind lever feels rougher than the M and can't be cocked double stroke (don't think, the "rough" is a big issue - you just can clearly feel and hear (!) the gears, when cocking).
I mostly cock my M6 and M7 with two short strokes, as I feel more comfortable, not to move the camera to much, while in my face.
It's film loading with the "fiddle leader into plastic slit" is much slower, than loading a late M with the tulip take up spool. Really, the M is loaded and ready in half the time, while I still fiddle with the FM (I guess, that comes down to practicing).
The rewind action on the FM3a is much lighter, smoother and faster as on my M6 and M7 (they require more strength, I slip from time to time and they feel more rough or even "broken" by comparison).
Metering the FM3a in low light is bad, as the viewfinder has no lighted indicators - there is nothing better than the M6 classic two arrow diode meter with glowing diodes, as the meter reading changes - hate the M7 meter. The FM2 should be fine with it's metering diodes.
Apart from comparing a SLR with a RF, the viewfinder of the FM3a is more dim, much smaller and a really good example of the "tunnel effect" (all in a bad way I am afraid).
The placement of the RF viewfinder on the left side of the camera back really is ideal for left and right eye shooters with both eyes open or one eye closed - I do all of this depending on mood, lens, light and subject.
Looking through the M is just easier and more comfortable.
Also, poking the wind lever in the eye is less bad on the M than on the SLR.
The all around fit, finish and feel of the M vs FM stands no comparison and is directly connected to their respective price.
The SLR feels indeed cheaper, but does the job veeery reliably. The materials of the FM3a's body are top notch as with the Leica - only brass, aluminum, nice leatherette. The ISO selector film window (yes a film window - love it!) and wind lever tip are plastic.
The ISO selector feels cheap, but is not worse in function as the Leica ISO selector on the back door (especially the M7 one).
The FM3a has both ISO speed AND EV comp interlocked in comparison to the M7.
The FM3a focusses up close even with the rather limiting 0.6m or the cheapo 50 f1.8.
The MD12 is absolutely gorgeous (if being loud is not an issue) - it is fast, precise, has a great grip, motor rewind and is good for many rolls on 8 standard AAs.
Shooting the FM3a on "A" with the MD12 is really quick.
If you're into flash - the FM3a has TTL (as does the M6TTL and M7) but is fully compatible to the much more affordable Nikon TTL flashes (Nikons top of the line SB-900 form today is almost half the price of the Leica/Metz counterpart).
The FM3a has a −1ev fill flash button, once pressed, giving an −1ev TTL flash underexposure, to just fill - nice touch!
The FM3a AElock button is a bit hard to use for left eye shooters (you've got the wind lever and your thumb on the AElock button in your right eye).
The Leica AElock on the M7 is easier to use, but worse, when shooting with several frames locked and motor (simply doesn't work with the M7 ;-) but with the FM3a).
I started, to have tiny FM3a with the superlight 50 f1.8 now in the bag together with my most used M8.2 and either a M7 for low light and tele or a M6 for a 15 or 28mm.
Thats a 3 camera - 3 lenses - 3 different film bag with reasonable weight and small footprint.
Every RF shooter ought to have a FM/ FM2/ FM2n/ FM3a with a wide/normal/tele/macro these days, as they are so affordable compared to anything from Leica.
It is a very nice camera indeed - RF shruffs should try one before complaining of sacrilegious naming of these items over here ;-)