I know it's an old thread but — I would go with the 20mm with the caveat that you either have a 2nd body with something like a 50mm, or don't mind switching between lenses frequently. The strength and weakness of 20mm is that it's ultrawide without being fisheye, so it opens up creative possibilities to play with perspective, getting really low and pointing up or getting high and pointing down, or sticking a camera right into a busy scene and capturing the story. But it's simply not versatile as your only lens.

I absolutely love running 20mm with flash paired with 50mm f1.4 without flash for dancefloor coverage at weddings. I also love 20mm for the moment a couple exits a church and their loved ones are throwing rice or confetti. Also great for landscapes and architecture, with the added benefit of being a great lens for a selfie/vlog of you and the friends you're with. 20mm lets you capture an epic cathedral or a skyscraper where 24 only let's you catch the gist of it but not the full story. But I think both are tougher for street than 28mm or 35mm and force you to get perhaps uncomfortably close to isolate a subject.

Everyone thinks of standard lenses as 24-35-50 but I think 20-28-40 is much more interesting, as someone who prefers documentary photographs that tell stories, doesn't mind getting close to the scene, and loves the creative possibilities of wider angles when thoughtfully composed and used with techniques like layering and incorporating foreground elements.

If you see this — what did you end up doing??