We also took our son hiking in the Baby Bjorn One last summer and haven't yet bought a back carrier for this summer.
From the research I've done it looks like the Osprey Poco or Deuter Kids comfort will be the best options for us.
I would recommend going to MEC or other store that carries them to try how it fits you, even if you will buy used from somewhere else.
When I last checked a few months ago there were several used ones available in excellent condition for low prices on Kijiji/Craigslist (in Toronto, but would probably be similar in other areas, especially where there are more hikers).
If you aren't going to use it often, you can also rent MEC-brand carriers from MEC for $12/day / $60/week (not all locations have them available).
Last edited by dpdp; 05-27-2020 at 08:35 AM.