A lot of developers prefer the mt76 due to it being more open. This allows them to fix bugs or do enhancements that can't be done in other chips.
I ran the r6220 for a while and found it to be flawless. I gave it away to a family member to replace her wrt3200acm, which has firmware bugs that can't be fixed.
After my hands on experience with the Netgear r6220 I'd say go with that. You won't find another one that is better supported.