Basically screwed either way. You can actually get high end Nvidia GPUs and monitors. Getting a high end free-sync monitor, pretty easy. Getting a high end AMD card, not so much. Either way you are locking yourself down to either vendor (unless you don't mind selling monitors and GPUs regularly)
Free-sync monitors tend to only work at fairly high refresh rates, so the minimum free-sync range is something to look for. Not much use if Free-sync doesn't work when your GPU can't keep up. Free sync can work over HDMI and DisplayPort. Since the normal scalar is used, the monitor may also offer DVI and VGA ports if that matters to you.
G-sync monitors usually have a good range, down to around 30 FPS. So they'll keep it smooth even if you are...