There has been considerable debate on the relative merits of the 5 and 8yd kilts.

If you consider the original kilt (aka the great kilt), it was made from a single piece of approx 8yds of wool, just over half of which was wrapped round the waist to form the kilt part with the remainder worn round the upper body. Eventually the 2 parts started to be worn separately as kilt and plaid, so the 4/5yd is probably more authentic in strictly historical terms. There is no real documentation on how this then developed into the 8yd kilt, but the most logical reason I have heard is that clan chiefs and gentry sought a superior quality kilt to denote their higher social standing and used longer pieces of material, eventually creating the 8 yarder which, of course, the lower orders then aspired to.

In terms of fit, appearance, "swing" etc the hand-sewn 8yd is far superior to the machine-made 4/5yd, but against that the 4/5yd is much less expensive and therefore for a lot of people more likely to be worn casually (risk of damage to the kilt balanced by potentially far less damage to the bank balance), leaving the more expensive 8yd for semi-dress/formal occasions. However, one of the principal downsides to this convenient arrangement is that, being approx. half the weight, the 4/5yd "lifts" much more readily in a wind.