Although Leica gears are generally worth more money wise than Nikon gears but the FM3a is quite a collectible camera and I think it's worth more than the SL2 which I don't think many people would want it.
I would keep the FM3a and get some Zeiss lenses. I have handled the Leica SL2, they are really nice cameras and the optics are insane good. That being said, their are more Nikon techs then there are Leica techs especially those who fix older Leicas like SL2s. Bear that in mind.