I don’t have a Mac. Does Spotlight handle units as well as Google does? Also, doing the calculation in Google is handy because it’s easy to share in a modifiable way.
Wolfram Alpha is pretty good and is more powerful than Google Calculator, but I’ve found Google Calculator to be more responsive and consistent and thus a bit easier for regular usage.
I don't know how well it handles units vs. Google but it does convert as you type, say "5inches" immediately shows it in cm but you can keep typing "5inches in m" and it'll do the math too.
