As a writer, I'm restless: always moving on to new worlds, new characters, new ideas. I love short stories in part because it enables me to take a snapshot of a concept - for instance, a flying city populated by people who believe the world below has been destroyed - and play with the thread for a bit before setting it aside and, like a child with crayons, merrily clutching for the next. When it comes to editing for short fiction, I have a definite (if not always consistent) tipping point between when I'll overhaul a story and when I feel it's effort better spent on a new work.
I do this with harp, too: I'm always eager to try new tunes, and I would far rather pick up new sheet music than revive a forgotten piece from my older repertoire. And cooking: I try new recipes almost every week. I rarely go back.
On the other hand, I have a certain nostalgia for old concepts, characters and stories. I'm an incubator at heart, so these tales that have had years to mellow from their writing have a powerful appeal. I'm also a perfectionist, so looking at my old flaws, from awkward prose to questionable plot twists to cliche worldbuilding, I want to fix that ... and I'm also intrigued by the cascading changes that stem from making those improvements.
So I find myself caught between the two. Should I try to salvage every story, or is it all right to decide that it's better to take what I've learned and spend the effort on a new work? Should I go back and rewrite old novels, or is it better to mine newer, fresher ideas? Is either extreme lazy and undisciplined? Which one? How in thunderation do I know?
And please, don't say, "Choose whichever appeals the most to you." Oh, if I knew that, I wouldn't continuously dither about it. Sometimes, it comes down to my sense of what might be more marketable, but that's always a best guess.
It's a work in progress.