as hamish said it is about a different manner to make the reef knots from the tartan. while worsted wool is a finer and more expensive weaving mill, twill looks a bit coarser, ruder. twill usually was used during the world wars, benefiting from its robustness. jeans for example are also made in the twill manner.

i got one kilt made from twill wool and i m as happy as with my worsted wool ones. the difference is as suggested granted and both hold their pleats perfectly. i wouldn´t mind about the difference as it is quite minimalistic.

considered that twill means in german "drillich" and wool is "wolle" you could see pics from both weavings on this page, sorry that it s in german, but just the pics there...
http://www.marquise.de/de/themes/howto/stoffkunde.shtml

@hamish: as i m in search of a really heavy kilt like more than 20 oz, would u say they are made all from twill? does anybody know where to find those kilts?

regards