(get-in order [:lineitems 2 :quantity] 1)
Contrast with the latter, where the get-in function just treats the subject as a nested associative structure. It can handle the null checking, and it doesn't care what my keys look like.
The only real advantage I see from the second line version is that I can write that code and run a partially completed program, that for example doesn't have a notion of orders at all -- and the program can still run. But I guess this just boils down to preferences.