$100 - Premium for new monitor with G-sync module vs exact same new monitor with no G-sync module
$0 - Premium for new video card with Mantle support vs exact same new video card no Mantle support
Depends on the variables. Are you already AMD? NVIDIA? Want 120Hz? Do your framerates vary a lot? Etc.
Going G-SYNC looks better than going SLI with a $400 extra card.
Fluctuating 50fps with G-SYNC looks a lot better than fluctuating 75fps without G-SYNC.
Remember, most of the time, adding SLI doesn't decrease GPU input lag much; the frame rendertimes are usually constant during alternate-frame-rendering in many games. It also introduces potential microstutter issues from GPU framepacing, though NVIDIA has done a great job of that. For many games G-SYNC would decrease input lag MORE than going SLI, too.
+$400 -- going SLI
+$150 -- going G-SYNC (if already NVIDIA)
Obviously it can depend on the game and variability of your framerates. In this case, since G-SYNC upgrade alone looks better than going SLI alone, the G-SYNC investment is better in this case. Sub-$400 graphics cards are DOA on a ROI basis, even $400 graphics cards wouldn't improve motion well enough in several video games to be better than G-SYNC. Assumes, of course, you already are NVIDIA and 120Hz and G-SYNC upgradeable, or was planning to get a 120Hz monitor either way. Or if you're building a cheap 120Hz rig on limited budget and can only afford G-SYNC -or- SLI.