Your comment prompted me to test both of them side by side. Actually,
Alt+0186 makes the larger circle (I bumped the font size up to 48 to examine
them); that is the difference I didn't notice earlier... one is not higher
than the other, only larger (the top of both symbols are located at the same
height position; it was the size difference that made one look like it was
higher than the other one).
As it turns out, though, neither one of them is exactly the right one to
use.<g> The symbol produced from Alt+0176 is the same height as the
apostrophe and quote mark symbols, but its line thickness is just ever so
slightly little thinner than them. On the other hand, the symbol produced
from Alt+186 is larger than the apostrophe and quote mark (about 25% so) and
its line thickness is just ever so slightly heavier. At normal font sizes, I
kind of think Alt+0176 looks a little bit better (but that is a highly
subjective opinion).