Not to be confused with the original SeaCraft 21, below are some details on the recent vintage SC21. It's a different animal than the SC20. 10 " longer, 6" more beam and 900 lbs heavier.
http://www.bustersmarine.com/SeaCraft/seacraft21CC.html
I think you get 45 mph, and close 4mpg at cruise with either option. The etec is a bout 40 lbs lighter than the zuke. The gear ratios and subsequent propping allow the zuke to swing more prop, im not sure if its enough to offset the "pop" mrobertson mentioned.