I've spent a good bit of time in both the 23's as well as the new SeaCraft 21 and they are not nearly as different as the literature would have you believe. The 23/21' are very similar (think of the 23' with 2' chopped off the back of the existing hull). Neither the 21/23' have much in common with the 20'. Look at the weight alone and you'll see that the 20' is a significanly smaller and lighter boat than either of those two.
The 21' can take on much nastier conditions than the 20', significanly more freeboard, but you'll need a larger vehicle to tow the 21'.....and a bigger engine to push it. The 21' is a really nice boat that doesn't seem to get as much attention as it likely should and is layed out far better than the new 20's. If my current 20' was stolen, the 21' would be on a very short list of replacements.
As seen in the photos below, I have my 20' redone to mimic many of the layout features of the 21'. I simply didn't want to deal with towing the heavier boat. Costwise, it would have been about the same for me to buy the 21' or have my 20' redone. The similarities are pretty evident in what I was trying to do:
current issue SeaCraft 21' -
my 20' before completion -
