Steam tends to be faster than using the launcher. And if you have the game through Steam, using the Launcher to download the update can actually cause problems. I had a file corruption issue back in version 2.4 that was related to downloading the update via the launcher, and when Steam tried to overwrite that update with a new patch, it corrupted the ED app file.
So, now I always use Steam to update ED. The other advantage of using the Steam Client is that you can just leave it up and running, and it will begin downloading 3.0 the second it hits the Steam servers. No need to babysit the process like you have to do with a Launcher download.
Oh! And the 3.0 Beta download was just shy of 16GB. The final will likely be slightly larger.