HI, the main difference between a model for 3d print and say film for me is that I can dynamesh the hell out of it and it also depends on what the 3d print will be used for. For example, if I am going to be just making a '1 of a kind' print I won't have to worry about having undercuts or slicing up the model depending on the size of the print (for molding and casting) . All I have to really do is make sure the detail will be high enough and thick enough in parts like ears, fingers and even cloth so the details will read well whether its 3 inches tall or 8 inches tall. The size of my printer plays a part as well. I have a Form2 laser printer and if I want to make anything over 7 inches tall the mesh needs to have sliced parts. If I want to make copies of the model it is easier to slice the model into parts and where I can hide the seems for casting and molding, and like you mentioned, water tight is a must. For a mesh for film you don't really have to worry about undercuts or slices making copies or how thick most of the geometry is or will be, or whether it will look good as a miniature or 1/4 scale. The main concern is the quality of the geometry, how it will deform, clean UV layout, and all of the maps.
