Look and alignment are indeed different in the 5 / 7 / 9 / 11 models, with the 5 and 7 being closer to blade performance side and 9 and 11 being true mallets. Some people prefer different head shapes and alignment lines. As for the even and half numbers, those are differentiated by the hosel designs on each. The 5, 7, 9, and 11 will be nearly face-balanced for a straight back, straight through stroke, and the 5.5, 7.5, 9.5 and 11.5 will have slightly more toe hang for a more arcing stroke.

I think you may have some success going to the scotty cameron site and reading what they have written about each head shape. It has to do with how you stroke the putter and the face balance and stability they have engineered into each head. If you are into that type of stuff. I tried a 7.5 for a little bit this season and it was a great looking putter, very nice to look at and put a nice roll on the ball!