It's always a good idea to give them an easy job for their first crop, so Shetland would be a great choice, with a more commercial tup for subsequent crops. However, if you are wanting something more commercial for their first time, then, assuming that your shouthdown x shetlands are bigger than pure shetlands, what about Charollais?
For a very commercial lamb that's easy lambing, we have found our Charollais to be excellent. Slippery, easily-birthed very active lambs that grow really well and have super conformation. The only downside we find is that the fleece is so very close and short, the lambs can be a bit bare and suffer in cold wet weather - so if the weather isn't great at lambing it may be best to bring lambers under cover to lamb, and then put plastic jackets on the lambs when you turn them out.
A full grown Charollais tup is a big boy, though, so if your girls are not large it may be best to use a tup lamb.
Oh - and when the time comes, the fleece of a Charollais x (Southdown x Shetland) could be awesome - there'll be spinners on here very interested to have a try, I'm sure!