I would recommend going with 8x10. I found there's a whole world of difference between 4x5 and 8x10 with the larger size actually being easier to use. A bonus of the 8x10 is that you can use reducing backs to adapt it to 5x7 and 4x5, so one really gets three cameras in one.

You may be disappointed in the polaroid color film that's past date. I bought some expired type 59 recently and the colors have shifted way off. Its good to use for abstract work though.