Edit:
That gif above ^ is actually a bit misleading as it shows 16:10 as losing horizontal range over the 16:9 but actually the 16:10 would be the same width as the 16:9 but the 16:9 would lose vertical space compared to the 16:10. But, being that it's SC2, it probably suffers from the same Diablo3 zoom issue with 16:10.... grrrr.
The gif is correct, and yes SC2 "suffers" from the idea that the aspect ratio will be achieved by chopping off the sides of the image, instead of gluing on additional vertical information.
That's the trick with aspect ratios - it depends on the game, whether you will gain or lose information by changing aspect ratio.
For example, in a game like Bioshock, if you change the aspect ratio by adding vertical resolution, you will see additional vertical information. But SC2 is not like that, and apparently Diablo3 is the same way, where if you add vertical resolution, these games instead will "zoom in" by chopping off the sides to make the image more "squarish" so you end up with a net loss of information.
You can check on sites like widescreengaming, and they will rate the game as Hor+ or Vert- or whatever the actual rating is; that tells you if changing the aspect ratio will reveal more info (+) or chop off info (-), and which way it happens (vert or hor). I think, haven't looked at it in a while. Just know that it changes by game.
If all you do is play SC2, then 16:9 resolution is pretty much mandatory so set your 1920x1200 display to run as 1920x1080. That's definitely what I do, and I don't mind the letterboxing (black bars on top and bottom) when I play SC2, because I see more.