Hey Paul,
I can't say that the AJAs are the better Option as I never compared them side by side, but just my feeling says that the AJA 4K got less latency (and you can input 50/60 HZ)...
As a long term Phoenix User I switched to AJA 4K last year, when I upgraded my Catalysts to the recent Macs and definitely no regrets!
Best
Rolf