I had 50 1.8 and it has really great IQ when you look how much you paid for it. Probably it's the best IQ/price factor in Canon lens lineup. But it's not too well built. It's focus ring is not useful (too small and doesn't allow for operation when lens in AF mode). Having 5d3 I think, sooner or later you'd want shoot any movie no matter what it would be. 1.8 is useless in this area because of not useful focus ring.
Summarizing - it makes really good pictures but:
1. It doesn't have USM - really loud
2. AF might miss sometimes
3. Focus ring useless for manual setting
4. Weak, plastic butli
If above is not important for you, then take 50 1.8 because you'll be happy with it.