There will be as many answers as there are shepherds
I always reckon 1. Keep something you like the look of or 2. Keep the local breed
1. It's always nicer looking after stock you have a particular liking for or 2. Keeping the local breed means there's lots of knowledge about them to call on, they'll be easy to get hold of and to sell.
I used to keep Rough Fells. Beautiful, friendly, calm and hardy. But very big and wool not much use. Now I keep Shetlands. Friendly, hardy, many colours, good lambers and mothers, small, excellent wool. Not as calm as Roughs.