Either or, 40 profile will see your speedo reading slightly fast, 45 will see it read slightly slower (but consider tyre wear and the error in the speedo and it's really nothing significant at all).

Tire size calculator

Stock size should be 205/50/16.

Tyre fitters would love you if you picked the 45 over the 40 though... anything under a 45 profile is a pain in the ass to fit!