All breeds have their pros and cons.
Ones I can speak of personally :
Charollais - very laid back, easy lambing (usually, I know one member on here has had a miserable lambing with Charollais this year), friendly biddable sheep, tremendous growth rates and carcase. Downsides - rather large, can be quite bare - young lambs may need jacketing in bad weather, adults and lambs may get sunburn on thinly-covered or bare skin
North Country Mules - lovely friendly sheep (just don't get between a full-grown ewe and her cake!), usually pretty hardy, have good commercial lambs to Texel or Suffolk, tremendous mothers, very milky. Downsides - too many triplets, can put too much into lambs at the expense of herself, resulting in twin lamb disease etc if not carefully managed. Personally I like the Swaledale cross Blue-faced Leicester Mule and do not like the Blackface cross Blue-faced Leicester Mule - I find the latter flighty and unfriendly. Their lambs do have better carcases though, and their fleece may be better.
Texels - very commercial, should finish same year born, naturally make good use of copper so don't need copper supplements. Downsides - not always very milky so can struggle to rear two lambs, not always good feet, can suffer copper toxicity if they get hold of food or lick with copper in. Single lambs can be too large to lamb, so it's best to scan Texels and feed the twin-bearers (to make sure she has enough milk) and not the single-bearers (so the lamb isn't over-large.)
Beltex - top commercial breed these days, very motherly and milky, squat sheep but very very solid. Usually fairly laid back. Some people say they are ugly; they've grown on me - but then I like Muscovy ducks, too!

Downsides - pure-bred lambs can be too large in the shoulders and hips to get born. Feet not always good. Like your Southdowns, they maybe are a bit close to the ground.
Anything else I told you would be hearsay - and would include that the Romney is supposed to be a good healthy sheep, good feet, commercial carcase.
What about Shetlands, cross them to a Texel for a commercial lamb?
I'd been thinking about getting a Southdown tup for our hoggs, thinking it would be a nice small (but still commercial) lamb for a first-timer. Having said which, we used the Dutch Texel on our shearlings this year and his lambs have been small at birth, but really good shapes and good growth rates, so we will probably stick with him for first-timers.