I've always bought new/rebuilt ones because for the price difference to getting the rebuild kit, fighting with seized bleeders, cleaning them up so they look decent it's not worth the trouble of rebuilding them. That being said I have been known to keep the old ones incase they ever become hard to get or need any parts from them.

my motto is : "never throw anything away that you can stash on the shelf, or hide in a corner !"
and i wonder why i fall over "stuff" daily........