I have tried it with both to be honest and flexible works better if you have a slightly weaker defence. I used fluid once I was in my second season with Bournemouth when my defence was strong and everyone was capable with the tactic.
I have created this tactic and tested it out with Bournemouth, Wolves and Peterborough. It relies on quick wing play from wingers and also overlapping full backs. Also, the key is to have a good Defensive Forward, I have tried with other player roles and nothing seems to work as well as that...