I always say get three rather than two, if you can. Then if something goes wrong, you don't have an only sheep. Although if you're buying with lambs at foot, it's not such an issue.
Starting with two ewes with lambs at foot, especially if one or two of those lambs are ewe lambs, you will be amazed at how quickly your flock grows and you are having to decide which ewe lambs to keep on and which to sell! It's slow for the first year or two, but once the ewe lambs lamb themselves, flock size grows exponentially.
Shetlands are most usually not tupped as lambs, but I did tup some of mine as lambs and they all did brilliantly, apart from one who didn't hold.
I suspect that any individual is either the sort of person who loves the placid, cuddly teddy bears that are Ryelands, or is the sort of person who loves the chutzpah of the big-character-in-a-small-package that is a Shetland.
Both produce excellent meat and lovely fleece you can use.
One does hear of the occasional lambing difficulty with Ryelands, and much more rarely - almost never for pure bred Shetlands - with Shetlands.
If your land is limited then you may not want to be running Shetland lambs on over winter. Ryelands should be able to fatten in one season, if your grass is reasonably good. Personally I would ring all male lambs and not have the worry of entire males, even with a breed fhat should fatten in one summer, but some people wouldn't bother to ring Ryeland tup lambs.
Shetland tips are horned, and male lambs therefore are also horned. Baby horns can get damaged, caught in stock fence, etc, and can be prone to getting struck by flies when damaged, so do create a bit more work.