Both, the NAD M3 and Musical Fidelity A5, are excellent pieces, you can't go wrong with either.
I listened to an A5 set-up using the new B&W CM7 floorstanders, and I believe a Musical Fidelity disc player. The sales person played a Diana Krall disc, the sound was so sweet and clear, detailed sound-stage, and very transparent- absolutely beautiful sounding. Moreover the A5 does have a bit more power than the M3 (250W X 2 @8ohms, as compared to 180W X 2 @8ohms; 24dbW compared to 22.5dbW).
While I have'nt listened to the M3 I have listened to the M15/M25 pre-amp/amp combo, one of the sweetest sounding (multi-channel) set-ups for the price. All the M-series components are excellent. Excellent build quality.
If it were me, and this is to serve as your primary system at home, I would consider a decent multi-channel amp (like the M25), and combine it with your receiver (as pre-pro). This way you can graduate to a dedicated pre-pro later (NAD M15, NAD T175, etc.,...). A good start, and provides for a more versatile system (good 2-channel and multi-channel capabilities), can use for HT with receiver and great music source.
However, if you really only care about optimized two-channel performance I would go for the A5 (or the M3) and a good disc player.