Buckles and I prefer two. Although I do have kilts with 3.

That said, for quick and casual comfort, I can see why people would be drawn to velcro. I would imagine they can also be adjusted over a more varied range of waist sizes.

Certainly my personal preference for buckles is not a judgement against any other means of securing your kilt. In fact, I have heard of those that have absolutely no closure whatsoever and rely exclusively on their belt to keep their kilt up. Again, not my preference but I find it fascinating.