Depends if you think motion blur is an issue or not, I'd say G-Sync mode is far better in most circumstances but ULMB is great for playing stuff like fast side scrolling platformers.
G Sync shows the frames as they are rendered and gives great & more responsive (than non g sync) visuals with no tearing because it's not trying to update the screen X (say, 60) times per second like a regular monitor, it means things with varying framerates feel smoother and look nicer.
ULMB scans the light behind the screen on and off constantly to work in conjunction with the screen and your eyes/brain to make you see the images more distinctly, the downside is that you lose brightness because the light is turned off just as much as it is on but the upside is that moving objects don't blur as much as they normally would, the downside is this mode is not using G Sync and so it is beholden to all the tearing and other problems you face in a standard monitor, though these are lessened if your game is running at a consistently ridiculously high framerate anyway.
That's my understanding of the two solutions and why you would use one over the other in different circumstances. For me personally on an Acer XB271HU I use G Sync about 99% of the time.
