Spiral Calendars are the less traditional choice than their stapled counterpart but they still result in a quality product.
A spiral bound calendar's pages don't need to be printed in multiples of 4 like a stapled calendars pages do. This gives you more versitility in the amount of pages you want in your calendar and will also allow you to print a calendar with an odd number of pages.
Unlike the stapled calendars, you can also choose to assemble the calendar with a plastic cover and vinyl back instead of just a heavy cover. You are also given more variety in the binding as you could bind on the short or long side of the page.
Keep in mind when printing a spiral calendar to leave at least a .5" binding margin on the pages for any content you don't want the spiral coil to go through.