Depends. A 120 Hz monitor will give you smoother gameplay if your GPU can keep up. However, you really only notice a meaningful difference in fast-paced games, mostly first-person shooters.
1440p will give you a clearer picture. It will also allow you to get bigger monitors without sacrificing clarity. It's hard to pick out the increase in picture quality when you first switch from 1080p to 1440p, but once you get used to 1440p going back to 1080p is actually quite jarring. I've had 1440p monitors for a few months now and when I go back and look at 1080p, I find it to be of noticeably poorer quality.
As for 3D, I'm not a fan. I think right now it's a gimmick and not a lot of games support it anyway.
They have different characteristics. What looks better is personal preference.
3D Vision brings things to life with full binocular vision. That means a lot more to me. Not everyone seems to care about 3D, or gets sick/headaches from it. With the Helix mods, over half my games support 3D well. Most the biggest blockbusters these days have it built in (Crysis 2 & 3, BF3 & 4, Metro 2033 & LL, Tomb Raider...).
120hz makes things feel responsive and smoother, but 1440p gives higher definition and usually better color production.
If you are more into performance and smooth fps then 1080p 3d is the bang for your cash while if you want a crystal clear gaming experience go for a 1440p (Sacrificing your fps if your gpu is not powerful) therefore in my opinion the 1080p 120hz monitor is the best at this moment