I want to buy StarCraft II on the Blizzard website, but I want it installed on two different computers (both Macs). I'm wondering if I can install it on two different computers through Battle.net, or am I better off just buying a hard copy of the game and doing it that way?
You can install Blizzard games on as many computers as you want to, however - you'll only be able to log into the game on one PC at a time. This means, for example, that if you have a desktop PC and a laptop you use when you're away from your desktop, you could install the game once on each machine and log in on one of them to play the game. Once you've logged out from one, you can log into the game on the other.
StarCraft 2 is 15gb in size when downloaded from Battle.net - this excludes any patches that will need to be installed after the game has finished downloading. You would be able to install it on one PC and then copy the installation folder over to the other. Once you have completed the copy, manually start the launcher from the game folder and select "Repair Tool" from the system bar.
It is worth noting that if you purchased a physical copy of the game, you'd still have to add this to a Battle.net account and this account would still limit you to playing it on one PC at once. While you wouldn't have to download 15gb to install the game, you would still need to install all of the patches once the game is installed.