120 hz will be smoother, the 24 frames will have judder with panning, the 3;2 pull downs are to try and create a smooth image for 60 frame tvs, as 24 frames doesn't go equally into 60, so they have to do multiples of three then two frames to pull it off, but you get the judder because of this. Its actually a pretty big difference when you see them play side by side, however, 24 frames has a distinctive look that makes movies kind of surreal as the motion does exactly mimic real life like 60 frames and higher would. Its hard to describe, but if you pay attention you will notice it. That's why they call 120 hz soap opera effect because it mimics tv shows which are done in 60 frames and doesn't look professional for what we are used to seeing with 24 frames. I will sometimes watch movies in 120 then at another time 24 frames because the experience is different, although I watch more with 120 as I really like the smoothness. In fact when I watch in 24 frames it takes me a few minutes to adjust to fast moving scenes as they are just simply more blurry to look at. I guess it comes down to preference, but I'd say most prefer 24 frames as it gives the movies a slightly different feel than what you get with broadcast 60 or added 120 frames. With real fast scenes and very high 120 hz, sometimes it can create its own distortion, that is just as bad as blurriness, but if you keep it on low it gets rid of it. From what I remember, movies were made in 24 frames originally only because the new addition of sound couldn't be synchronized at a higher frame speed, so they stuck with that even to this day.
I'm curious, what kind of 3d setup are you going with? a passive? which projectors?