I've got both. I bought my A20s about 1/3rd through my ppl training and are my go to headset. I recently ordered two Zulu 3s for my wife and young son, just couldn't justify A20s for them (I saved $500 with Zulus over two A20s, to put it in perspective). I have gotten to test both, and I prefer my A20s for me. In my own testing I can tell the noise cancellation is a little better on A20s, and they feel better on my head. However the Zulu 3s are nice, real nice, and am very glad I bought them for my family. I love the Kevlar cable that doesn't kink, but they do seem to squeeze my head more. Lightspeed customer service is great, I ordered directly from the factory and they also sent thicker headpads, which I put on for my 7 yr old.
Anyhow you can't go wrong with either one, you really should try both imo and see what you think.