But the problem is that 20mm have better picture quality.
And the 24/1.8 is better than the 20. How important is "better picture quality" ? What does it buy you ? The 20/2.8 is really only a mediocre lens to start with; probably not that much better than the 16-50. The 35 is going to be sharper, but do you care ? If you're mostly shooting video, you may not see the difference between two lenses. And for your vacation shots, what are you doing with them ? Posting them at 100% for people to pixel peep or printing 30x40" prints ? (And if so, the 20/2.8 probably wouldn't be the best choice)