if your gpu is capable of delivering 120+ fps, then the gameplay will be smoother. You will notice the difference, no doubt. If your gpu cant do 120 fps, then dont bother, because you wont be able to enjoy the full experience of such a monitor.
the 3d difference between 60 hz and 144 hz is much more observable indeed.
PS : professional games often suggest having as many fps as posible, even by lowering quality. they would not recomend 120 + fps if they had a 60hz monitor.