Stimmers are fine for doing the fiddly bits, like op against walls, fences and around trees (guarded) etc, but you will not get a good finish with it like you get with a mower.
Unless you have a postage stamp of a lawn, I'd invest in a mower!
I usually strim first then mow, but I usually only need to strim every other cut.
Unless your into fancy stipes, you'r probably better off with a rotary rather than a cylinder mower, as they are more versatile.