2D girls are mostly like the open source versions of 3D girls; free and easy to use, modify, distribute, and duplicate as long as adequate credit is given. While 3D girls are often bothersome, buggy, expensive, and require constant attention.
A one piece is the patched version of the two piece, very few exploitable openings. A two piece can have serious 'wardrobe malfunctions' these malfunctions are actually “features” features I am interested in exploiting.