noob question: When I click on a large game on my library, and then on 'offline backup game installers', I get several chunks to download (part 1, part 2 etc). This doesn't happen with Galaxy.
So am I expected to worry about several downloads instead of a single click, if I choose the 1st way?
IF you want to get proper offline installers, yes. (You can also download these via some sort of submenu in your Galaxy library; I'm not sure if doing it this way downloads all the parts simultaneously, or if you still have to click on each one.)
One reason they split them up is so that, if one file gets corrupted during downloading, you won't have to download the entire game again (some games these days have downloads of more than 30 gigs), only the affected "chunk".