I scored a Diana instant back at urban outfitters for 23 bucks. Keep an eye out the sale bin at that store. I prefer Diana, because I prefer the Diana look. You can't fit a Holga instant back to a diana camera. They are different cameras. (I didn't know Holga even had an instant back)
different cameras, different films. I'd buy a $5 polaroid "colorpack" which has a meter before I'd buy an instant back for one of these cameras. Larger prints, good viewfinder, much cheaper. That said, the colorpack doesn't have a soft look with falloff that the two you mentioned have.