Ted, there is no difference really, other than the obvious. There might be about 3/4 of an inch difference in length, actually (just measured a couple of waistcoats), but hardly any big deal.

One would wear the same neckwear with either: a four-in-hand neck tie.

The key here, Ted, is not to get hung up on pretty insignificant details.

BTW, almost all of my "non kilt" waistcoats are 6 button: the two that are 5 button are exactly the same length.