Upload & Sell: Off
Yeah, it's personal. But look for these features:
Live back (adjustable resistance for leaning back to help support your back)
Adjustable seat depth and angle
Arm rests that not only go up and down, but that swivel to support your arms/wrists when typing
Aeron - meh
Steelcase Leap - excellent. Had one at a job 6+ years ago, and bought one for my home office, too.
Herman Miller Mirra - excellent.
Standing up part of the day is not a bad idea. You can do a DIY build on a standing/sitting desk using parts from IKEA. There are also standing desk units available that just sit on top of your regular desk.
Look for used office furniture places in your area for deals on higher end chairs.