I've just been looking for chairs. My advice is to go to a back-care store, not an office place, and get a consultation. I paid £50 for a 2-hour consult with a qualified ergonomist (which I get back off any purchases from them).
He gave me a bunch of great advice on how to use my mouse and keyboard so that it doesn't wreck my shoulders and wrists, and we tried about 7 different chairs. Gotta say, I wasn't impressed with any of the HM chairs - they look great, and they're very comfortable, but they don't support you as well as many others.
I found that this was the best chair for me in the end:
xenium–duo back® - Split back rest, double comfort for your back
Costs more than the HM chairs, and doesn't look as sexy, but far better to sit in.
(From Germany - you know the Germans always make good stuff

)
I wouldn't necessarily recommend it for you though, because you probably have a different
body shape to me. The important thing is to get a proper assessment and recommendation. You spend at least 1/4 of your life sitting in the damn thing, so it's worth a bit of time and effort to get right.
Not ideal I know, but I also don't want to spend a day just to try out some chairs. That time can be spent making money.
This is classic short-term thinking. You're not choosing wallpaper for the spare bedroom here, where it won't matter if you get it wrong. If you do a lot of keyboard work, you're at risk of RSI injuries if you don't get your set-up right. An RSI injury can render you unable to work at full capacity for MONTHS, and in the worse cases, require surgery ( I speak from the experience of people I know personally ), A couple of days spent getting it right now will save a lot of pain later on. If you need to catch the time up, ban yourself from WF for a week or two.