Here's a link to Polaroid's film web page: http://www.polaroid.com/products/pro...US&sc=Packfilm
The SX-70/Time Zero and 600 film are the same size. The image size is 3.1 x 3.1 inches, a square format. The older pack films (type 80 series) were 2.75 x 2.88 inches. The 66X series pack film image size is 2.88 x 3.75 inches.
I'm partial to the SX-70 myself because I believe it is a better camera than the 600. But, that's just me and I also favor manual focusing and exposure settings. The SX-70 is a true SLR. Its considered to be the best camera Polaroid has ever made. There's a method for adapting the 600 film pack for use in the SX-70 but I've never tried it. As long as Time Zero film is available, I don't see the need of bothering with type 600.