Both brands are about the same in terms of support longevity, and somewhat par for the course in the SMB segment -- typically only a handful of firmware releases over two, maybe three years, receiving just enough development hours to make their added code (on top of very similar OpenWRT innards) work as close to advertised as possible. Then they tend to move on, usually lagging slightly behind the newest wifi standard. Almost consumer-like in their cadence. If I had to give an edge it might be to Senao (EnGenius), as I prefer their work over the years to that of TP-Link, but overall, not a whole lot of difference if you just look at what they have out there today.
For better support in the same segment, you'll want to look to vendors like Cisco, an example being their Small Business wifi APs, like the new CBW series (CBW140AC, CBW240AC, etc.). They tend to push out a half-dozen or more updates over a multi-year span on that class of gear (typically 5-7 years of support, sometimes longer). Or, if you have the technical chops to self-support, any Qualcomm hardware that is OpenWRT compatible -- many APs have received 10+ years of OpenWRT support. Also, certain series of Ubiquiti or Mikrotik products; they might not have quite as much support longevity as Cisco stuff, but definitely longer then most TP-Link or EnGenius gear.
For much beyond that, you'll need to look to enterprise stuff -- Aruba, Cisco Aironet/Catalyst, Mist, Ruckus, etc.