My arcade machine had an original lo-res (standard) monitor.
That is designed for:
Horizontal ------------------- Vertical
Scan Line: 456 Pixels Screen: 262 Lines
Resolution: 336 Pixels Resolution: 240 Lines
All the games I wanted to run on this are older "vertical" games (PacMan, etc), so it wasn't an issue for me.
The Medium res monitor is designed for:
Horizontal --------------- Vertical
Scan Line: 646 Pixels Screen: 312 Lines
Resolution: 512 Pixels Resolution: 288 Lines
However, I would think MAME would be able to run a lower res game at that resolution (with stretching or centering or ??).
Higher res games coudl probably be downscaled..
But I don't have much experience with MAME and resolution scaling.
I actually went with an emulator called Vantage as it was optimized for the games/monitor/CPU (OLD AND SLOW) that I was using.