Suit or Tuxedo
This second most common question about men's wedding day attire is more easily answered when you rephrase the question. Formal or informal? Traditional or modern/contemporary? The answer is now probably a lot easier to figure out.
A tuxedo just can't be beat for making a guy look his very best. While today's suits can look amazing, they are still less formal and will never lose that business look. For many couples, a suit simply does not reflect the importance or grandeur that is a wedding day. This is why you did not see Prince William in a suit on his wedding day. It's simply not done.
That said, there are very few getting married today who run in royal circles. Thus, a lot of freedom and personal style choice can influence many a couple to decide on a suit over a tuxedo for their wedding day. Keep in mind that a custom tailored and fit bought suit will get a lot more use than a take-it-as-it-is rented tuxedo.
Tuxedos as well are more expensive to purchase than a suit, which is why most end up renting one. The problem there of course is one of fit. The rented tuxedo will have either the wrong length in arms or pants, never mind fit across the shoulders. All you have to do is look at a 1/2 dozen or so wedding parties to easily see which men have rented and which have opted to purchase. A rented tuxedo will never fit, and therefore never look as good, as a tailored and fitted one.
There is also the difference of fabric and finish to consider, the informal vs formal look. This great video is the best explanation of that difference.