I should have done a search before placing my post this morning. LOL! Anyway, I have the same boat, and have tried a Mercury Enertia 19, and yesterday I tried a Stiletto Bay Pro II.
From my research, the Enertia gave me 2 mph more than Stiletto Bay Pro II. I really liked the pull on the Stiletto, and it is about $200 dollars less than the Enertia.
What kind of speed and rpm's are you getting from the 21? I am thinking of going to a SS 21.
As an FYI, the 19 aluminum pulls me out of the water on a wake board, with 3 adults in the boat, and I'm 220 lbs.