The more you're able to do, the more valuable of a player you are. Lefties can be limited in the positions that some coaches will let them play, so I wouldn't be giving up anything just yet.
The older age groups of FP softball are littered with kids who pitched at some point, my DD included. I'd explore the pitching until you determine whether your DD is a pitcher while still being able to occassionally catch. If she hasn't cracked the lineup somewhere as a regular pitcher by 14U, it probably won't happen. Even if she's pitching regularly, that doesn't mean that she can't soak up some innings behind the plate when needed.
She doesn't "have to choose" if someone's going to play her in both spots.
But I do think at some point she'll have to commit to one or the other if she wants to really develop.
There's also this: P and C are both incredibly physically demanding positions. Will she be able to handle catching a game and then pitching the next game? It's one thing to catch an inning in a pinch. Good to be able to do that, I suppose. But it's another for your two positions to be pitcher and catcher.