Having owned both FM (actually three, two FM and one FM2n) and an M6 at the same time. The FM is the more versatile due to it being an SLR ... access to long lenses, no problems with focusing close up and parallax, more available lenses at lower prices, etc etc. Both have decent built-in meters. Neither are perfect. Both are durable and useful cameras, and both are excellent cameras capable of taking excellent photographs.
Both are long out of production and sell for relatively modest amounts of money now. Lenses of all kinds are available for both at whatever premium price (or bargain price) you want to pay.
Pick the one you like and that does the job for you, that's all.