It really depends on quite a few factors, like cost, storage space, boating conditions, amount of planned entertaining.
It's just my wife and I too. We went with a 212X, and it's plenty of boat for what will typically be two people. We boat on inland lakes, and entertain on the boat occasionally, but not a ton. The 21 foot boat has been plenty for us. If we entertained more or were boating on the coasts, we would have likely gone with a 24 foot boat, but the 21 foot boat has been great. Also try to look at an overhead shot of both boats, I know they are floating around this site somewhere, the majority of the extra length is seen in the bow of the boat. The main cockpit area is similar in both boats.
The width of the boats are also the same, so going from a 19 foot boat (8'-0" Beam) to a 21 foot boat (8'-6" Beam), makes more difference than a longer bow seating area.