Since you remark that your monitor is 120-Hz-capable you're most likely already aware that you can get 240-P modes by way of MAME's black-frame insertion feature (already discussed and explained here; never tried it myself). For the systems where MAME's emulation won't do, Retro Arch may help, since it also contemplates the feature (not sure about Amiga series emulation, though), so a black line generator is rarely necessary for emulators with a monitor like yours. I tried one of those with a DC and some low-res games and didn't like the way it looks at all (Compaq P-1220), but it's a matter of how used your eyes are to 15-kHz displays, I guess.
I wouldn't worry of which version of GM fits better for your specs more than of which games you want to emulate with those. If you're facing all the troubles of configuring a system for advanced emulation, you'll regret to not have spent some extra cash for a little more RAM or a better graphic card.