If you're discussing a topic then 100% commentate after recording video, it'll be easier to talk about the topic you want to and can read of a script or whatever helps you..
Only record over gameplay if you're reacting to it or creating a playthrough etc.
I might be a bit hypocritical.. I've personally recorded commentary while playing games to jog my memory, as long as you don't talk about what's happening on screen.
Really though it is opinion, do what makes you feel comfortable and records the best commentary