What are the characteristics of wool and worsted spun yarn

Update: 24-05-2021
Abst: Worsted spun yarns are part carded semi-worsted yarn  before coiling into a can and then passing through gills, which align the fibres, and combs, whi...
Worsted spun yarns are part carded semi-worsted yarn  before coiling into a can and then passing through gills, which align the fibres, and combs, which remove short hairs, coarse hairs and vegetable matter.  The result is tops, which are then re-gilled, auto-levelled and split for worsted spinning.Semi-worsted spun yarns are gilled and worsted spun, but not combed, so the wool is more completely in the character of the fleece but the yarn is smoother and stronger than if woollen spun. It is also possible to woollen spin combed fibre, which makes nice soft but bulkier yarns. These are refinements we play with for NFC customers to improve yield and handle, depending on the fleece type.So worsted spinning will make people think the yarn is softer and will produce a finer yarn, but you have two possible downsides the yield from greasy fleece is much less and there is a greater risk of pilling from any remaining short fibres or shedding from smooth fibres.
A woollen spun yarn will generally pill and shed much less, even with a high proportion of short fibre.These are examples, firstly of woollen spun and then of worsted spun Jacob wool, illustrating the leaner yarn with higher stitch definition on the right and the fluffier yarn on the left.Short fibre cannot easily or economically be worsted spun  on average a staple length of around 4”/10cm is needed, while woollen spun yarns can be made with fibres averaging 2”/5cm or even shorter if making a thick yarn. So sheep with short fleeces, like Southdown, are less good for worsted spun yarns. Generally coarse fibre will benefit from worsted spinning as it will feel smoother than if woollen spun and lustre fibres will have more sheen if worsted spun, thus will dye to a more brilliant tone as well. 
A woollen spun yarn will have a greater halo, while some worsted spun lustre yarns can look a bit hairy, though they will feel soft, which is why we like the woollen spun combed Cotswold and woollen spun Wensleydale options.I personally do not think Shetland benefits much from worsted spinning, as the best fibres are soft and light and bulky, but Jacob improves noticeably with worsted spinning. It is interesting that woollen spun Jacob using the whole fleece will produce a grey-brown shade, while worsted spinning brings a browner grey result due to the differing arrangement of fibre reflecting the light differently.I would not worsted spin North Ronaldsay as the removal of the coarser fibres also removes the character and anyway the softer fibres give a generally soft result.I like both the woollen and worsted spun versions, though the worsted spun is obviously softer. 
But I have tried some worsted spun BFL which is like knitting with cooked spaghetti and I personally like a yarn with a bit of character and presence for knitting, and for wearing.  But people are spoiled by the softness of Merino and worsted spinning, which is a shame as it’s really much more interesting to make yarn choices on other grounds than only the softness.It is traditional to worsted spin the lustre fibres such as Cotswold and Wensleydale and the resulting yarns are nice and soft and strong, but the fibre makes for a very lean yarn and if the wool is combed but then woollen spun you get the benefit of both the softer result from combing and the bulk from woollen spinning. Gotland will tend to shed a bit if worsted spun, like alpaca, although yarn is gorgeously soft, so it is one which would work best combed and then woollen spun to improve the range of fineness in the batch.