Myself, I wear the self-tie garters almost exclusively. My reasons are comfort, but also because the experience of tying my own garters seems somehow more "genuine" to me than using elastic. After all, I tie my own shoes, I tie my own neck tie, bow tie, etc. Tying my own garter knot seems to fall into the same category.
This is it for me too. I just like the old-school method of tying a garter instead of buckling an elastic thingy. It seems phony to me, like wearing a clip-on tie. I honestly don't understand why the native Scots have migrated to elastic bands and flashes, when other parts of the kilt have stayed so traditional and authentic.

But yes, either one (garters or flashes) are perfectly acceptable. It's just down to personal taste.