Sigma 20 1.8 vs Canon 28 1.8 ??

Hi there, I need some advice

I'm looking for a fast wide lens for portrait shots in available light when my 50 1.4 is too long.

I can't afford the fantastic 24 1.4 at the moment

I'd like the sigma 20 1.8, but seems that AF in low light hunts back and forth

Canon 28 is more reliable than the sigma, but with a longer focal.

I need to know, is that awful Sigma AF at available light or it is still usable, for example at weddings in room preparation?