When these discussions come up people often mention the Steelcase Leap as an equal. Find a local store that sells them and try them all out. For me the major distinguishing point was the lower back support. The cheaper Aeron version would dig into my back while the Leap had some flexibility to it. I'm not sure if the higher end Aeron chair would have been superior with the contouring lower back support but I'm glad I got the Leap over the base Aeron model.

No, you don't need expensive chairs for coding, you don't need to "sit down well" because it is not possible or good for you. Do sit for hours in a "correct" posture and you will discover that your body tries to move constantly from it (blood circulation needs your pressure points to change constantly or bruises will appear).

Instead of buying expensive things go buy experts book on the area of posture, or better, go talk with expert people in the field. Books like this helped me in the past(I had pain in my back years ago) : http://www.amazon.com/Steps-Pain-Free-Back-Solutions-Shoulde...

But I assert you that in the area you live there is already lots of good people that could help you. In my area I found a martial arts expert(that won Olympic gold medals when young) that could identify wrong postures from an eye shot.

PS: I code mostly standing up and with a high stool.

I do way prefer the Aeron over the Mira. The back support on the Mira is better but... Once I've used the Aeron's pellicle mesh (both in the back and on the part you sit on), I simply cannot go back to a chair which doesn't have a pellicle mesh.

The Aeron is a bit cheaper and you can find quite some at a bargain price if you're in the U.S.

I forked that and $300 or so on a Topre-switch based keyboard (an HHKB Pro 2 but YMMV, some people prefer bigger keyboards -- I'd rather move my hands / fingers as few as possible). Probably that a real split / symmetric keyboard like the Kinesis Advantage or the Maltron are even better (they're using Cherry Mx switches, no Topre).