Thanks, for the advice, I have both 5D Mk3 and 7D Mk2, I found that F4 hand held at night was a challenge and just felt I could do with a faster lens. Also would like to explore more with prime lenses.
Since you already have the 24, I would suggest the 20. I have one lens that does not accept filters and I have been able to work with it for many years. I hear you about the speed but, with today's cameras I'm wondering why an F4 isn't enough. Without additional information such as camera body and type of shooting you want to do, it's had to give any more advice. What is it that you need/want to do with your existing lens that it won't do for you? Best of luck.