The way your booklet is setup not only will effect the look of the piece but can also effect the price of printing and mailing. Typically we refer to the orientation as either portrait or landscape. Portrait is where the bind is on the longest edge and landscape is where the bind is on the shorter edge.
Typically printing portrait booklets is more cost effective, but it depends on the volume and size of the piece. When printing landscape booklets the size of the sheet is bigger which reduces the number of pages that can fit on the sheet and the printing presses that can be used for printing. This increased sheet size often increases the cost of printing, but it all depends on the size of the booklet and the quantity being printed.
The orientation of a booklet will also effect the location where the address will need to placed for USPS requirements. Especially for booklets smaller than 8.5x10.5 the USPS has very specific areas where the address will need to be placed, which may effect the design and layout of your booklet.
It is best to consult with your mail house and printer before starting your design to make sure that the design and print production align.