I have a 21SE with a 2000 Yamaha 150 two stroke like yours. I had the factory 17" pitch prop and I dropped down to a Propco 15" pitch 3 blade prop. Much better all around for my type of boating!
I ran a 21SE 150 Yamaha 2 stroke with a SS Powertech 3 blade 15. I never run with a light load.
If you always run with a light load and don't have deal with shallow water a 17 might be better for you.
I now run the same boat with a 200 Yamaha 2 stroke with a Mercury Mirage Plus 17.