+1 seamless. you'll double your install time trying to align two large tiles, as opposed to just slapping up one-piece.
get yourself a "BodyGuard Knife" from YelloTools. I used to do this by tearing-out the backing paper, but the knife makes it much easier to just slit-out a vertical 4-6" strip from the backing and have two people hold the corners aligned on the window and apply the center strip. You've now got yourself a "book-hinge" and easily finish the graphic in two halves.
+1 to a frosted vinyl graphic being the cat's meow for full window coverage. The images are softened by the transparency and most of the frosted films are opaque enough to block out anything on the other-side of the glass.
O, and I would check with the city/town if you're even allowed to be blocking-out the window. Most municipalities will only allow tinting up to a certain % and require a certain % of visibility from the outside-in. Easiest way to have a client refuse to pay, is when they're being cited by the city for unapproved graphics like these.