I edited the orignal "growing from the corner" pictures to make a better visual comparison, since the real size difference would grow from the center...

Note that while the 30" has +80px top and +80 px bottom vs the 27" -- the 30" does not have any more pixels across the width than the 27" , its pixels are just that much larger.

The 80px top and bottom size 'gap' would be much smaller size-wise at the same pixel sizes --> 75 inches top and .75 inches bottom (3/4inch top and bottom) 1.49" total to be exact if both were 108.8ppi.

Another way to look at it on more equal terms is that if you moved the 30" panel back enough until its width (and ppi) looked equal to the 27" to your viewing perspective, there would be .75" peeking out top and bottom (about the diamter of a dime coin) in relation to the 27" screen, 80px tall each.

120hz monitor hardware sends screen updates to the lcd panels every 8.3ms (120 times out of 1000ms per second.)

60hz monitor hardware sends screen updates to the lcd panels every 16.6ms (60 times out of 1000ms per second).

At 60fps, a 120hz monitor only has to change the pixels shown every 16.6ms (8.3ms*2).
At 61 fps - 119fps, a 120hz monitor does some repeated frames on updates. More 8.3ms*2 frames toward the 60fps end, very few repeated the closer to 120fps.

At 120fps+ , a 120hz panel sends new action/environment, animation and FoV data (a new more modern distinct frame) every 8.3ms.

The real response times of lcds (outside of grey2grey quotes) causes blurring of the pixels, plus the non-instant backlight strobing causes image retention blur on any lcd. I suspect that the blurring would be worse when you push 120fps+ to get newer, more accurate action data frames every 8.3ms, and I suspect the blurring would be less when you push 60fps or less since it would be sortof like frame doubling, only changing the actual pixel content every 2hz (8.3ms*2 ~>16.6ms) . I'd like to find out for sure someday.

The consensus is that 120hz lcd's "feel" smoother, and have around 1/2 the blurring on certain types of blur.
Unless you are pushing 120fps+ to the 120hz panel, you aren't gettings the full 8.3ms of "more up to date action" consistently,
and at 60fps or less, not getting any more recent action frames at all.

Currently 120hz lcd's are all TN, which have really poor image quality and colors compared to IPS panels.
When choosing a 120hz TN screen, you are not only losing the IPS quality, but in order to get more than the "feel"
and get more current action every 8.3ms, you have to turn down a lot of graphics eye candy on the most modern, gpu-stressing games to maintain over 120fps with a powerful gpu, and may not be possible at all with a weaker gpu.
Additionally, all 120hz LCDs are 1080p, which means your pixel density will be poor (large pixels - espeically the 27" model)
and your desktop rez low by comparison to a 2560x 60hz IPS.

120hz benefit vs screen tearing when not using vsync?
Another thing brought up is screen tearing, which 120hz theoretically would raise the boundary for as long as you kept your fps below 120fps. However in that case, as stated above, you are not getting new distinct action frames every 8.3ms per 1hz.
The lower your fps, the more frames are doubled across two 8.3ms screen updates. It would still feel smoother but not getting
the most out of 120hz for accuracy and more current action displayed. Its also worth noting that screen tearing is worse on some game engines than others, and alleviated by driver updates in some cases.. so could be considered somewhat a coding issue.

60hz benefit vs microstutter?
On the plus side of 60hz, you should theoretically notice less microstuttering, since the microstutters on most modern games are usually above 60fps when using a powerful gpu or espeicially a dual gpu. The frames update every 16.6ms on 60hz. So basically
you can't see more than 60fps on a 60hz panel because it has 60 updates out of every second (every 10000ms). Since you can't see more than 60fps to start with, any microstutter that goes anywhere from 100 fps to 61 fps (as an extreme)example should be unnoticable as its still above the 60frames you were able to see in the first place per second at 60hz of screen updates per second..Of course whatever fps you shoot for should always be high enough to allow for ordinary scene/action complexity dips and minimums regardless.
   In the case of 120hz you would have more screen updates per second so microstutter could be more noticeable since the ceiling would then be 120fps.   
    *** However, microstuttering is mostly noticeable when you are playing at framerates most people consider too low, for example 30 - 35fps. You would definitely notice a 30 - 35fps drop to 25fps but you might not notice a drop in the much higher ranges so microstuttering complaints might be a little overblown by some people, especially since its more prevalent in single cards with dual gpus that tend to hit higher framerates to begin with.