I have personal experience deploying Aastra and older Polycom phones for small installations. As Mitel owns both brands now, I’m not sure where the new phones will be heading. Mitel’s support model has left a lot to be desired. With current models, if you are going to do fixed station provisioning, I would use Aastra as the web GUI of the phone is reasonably straight forward and responsive, Polycom’s, not so much. For the Polycoms, I decided to only provision with TFTP on my local network. I think the sound quality on the Polycom’s are a little better than the Aastra but they are close. This can vary a bit model to model within a brand so best to ask around or test it out yourself. I do have issues with Polycom’s with live overhead paging as the mic is sensitive and tends to feedback quickly if not properly placed. Many phones may have the same issue in this scenario.
I’ve not tired using any of the provisioning integration options with FreePBX.
Maybe try to buy a used model off ebay and experiment with it before making your decision. This might help prevent making the costly mistake of getting a model that doesn’t perform as you like. If you don’t like it you can always turn around and re-sell it.