In general terms, Flagship Lounges trump Centurion, but it depends on what you want. FL's don't have 'spas'... and some don't have staffed bars. But definitely more space and more abundant food.

To me, the space constraints of the Centurion lounges can be a real dealbreaker. I know they've been trying to solve this with entry rules and enlarging clubs, but it's still an issue many times.

I've never had a space issue at any of the FL's.