There shouldnt be much loss in IQ going to 120hz per say. I dont have that much experience since I just purchased my first 120hz Viewsonic a few days ago(but have used them often in other places). My previous monitor is a CRT so, having gamed many times on a 60hz monitor, its almost torture and hurts my eyes.
Like, typically some complaints on them is bad black contrast levels and such, so it is possible you might sacrifice some of that. It is true IPS have better colors, contrast, more details etc. I hear the whole bad viewing angle argument often, but I dont know, I tend to be sitting down at my desk looking straight forward when I use a monitor, call me crazy. I dont obsess about image quality to much, id rather have a smoother picture.
It comes down to what you do on your monitor and what is important to you. In fact, because 120hz will produce a smoother crisp image during motion, you could argue that image quality in that regard is easily better on 120hz monitors.
There is a misconception that 120hz is strictly for gaming. 120hz makes everything smoother so....its helpful even on deskstop use, although maybe not quite as noticeable. I would say if you dont game, you might be happy with an IPS instead if you really value quality over anything else(odd logic if you ask me).
And yes, an MMORPG has motion thus 120hz would be noticeable.