Without the ability to fit into a 24" space, your options are painfully limited on either TN 120 or IPS.
I'm really REALLY annoyed that IPS hasn't delivered a 120hz desktop display. It's been possible for a while now (since 2010) ranging from 15.4 netbook/laptop to 24", they just haven't evolved on the market. LG and Samsung have both premiered functional display units (including one LG that I want that virtually has non-existent display bezel).
If IPS and 120/240 were available, I'd say go for it. Since they're not, I do have to say that I've dealt with Samsung/LG IPS displays and they haven't been a problem in gaming for ghosting or the like. As with any, out-of-the box may require some mild to moderate adjustments for satisfaction, some units can do horribly out of the box but really shine when you fiddle with them. Same holds true for TN displays.
Course, the issue is compounded by the fact that there are different types of IPS, but find whatever is in your budget and size and look for as many reviews as you can, especially any that are properly done with applied calibration results as well.
For non-IPS, 120 is definitely much more fluid. Whether or not you can find a reasonable display out there under 24" is also questionable. 24" really is a sweet spot size when it comes to features AND resolutions, especially if you want better than 1680x1050, such as 1920x1080 or 1920x1200.