I'd agree with Rocky that since you already have some 5 yard kilts, may as well go for the 8 yard.

But I also wanted to comment on Kilted KT's comparison. It sounds like most of the differences he cited (5 yarder wrinkling more often, blowing up in the wind, etc.) would be due to the difference of fabric used in the two kilts. The amount of cloth used really shouldn't affect those things. But the weight and type of fabric will.

A 5 yard knife pleated kilt in 16 oz worsted wool is not going to wrinkle any more or less than an 8 yard knife pleated kilt in 16 oz worsted wool. The only real difference is that the 5 yarder will have fewer pleats and those pleats will not be quite as deep.