John: From Klaus audio http://www.klausaudio.com/subwoofer-box-articles/flared-ports-vs.-slot-ports.php
"Many people question what the difference is between flared ports and slot ports. There are fairly basic differences between the two, and depending on your situation, one might suit you better than the other. The common purpose of the two ports is that they must be the correct length for the particular subwoofer box to achieve the desired sub box tuning. There are a few differences that may help in your decision between flared and slot ports for your custom subwoofer box.
Flared ports are very good at virtually eliminating port noise. Port noise is the noise created by the friction of air moving through the port. If you have sharp edges on a port (as with standard round ports, which we do not use), you are going to have significantly more port noise than with a flared port. A flared port requires the smallest acceptable port area for a subwoofer box. Since the ends are flared, and the inside of the port is round (no sharp edges), it is unlikely you will be able to notice any port noise from the subwoofer enclosure. However, if you are running a large, high-powered subwoofer, you will want to use two flared ports. A single flared port is still capable of creating port noise when very large amounts of air pass through it.
With a slot port, you can use as much port area as needed for your application, which is especially helpful in SPL applications. A slot port requires more port area than a flared port to minimize port noise. We generally use a rule of 12 square inches of port area per cubic foot of internal volume of the custom sub box. You may go with a larger port area, but this will increase the total volume and length of the port. In turn, a large port area will mean an increase in the total size of your subwoofer box. Slot ports are the standard for our ported custom subwoofer boxes.
If you are not limited on space, we would recommend going with a slot port for your subwoofer box. If space is tight, and you have a few extra bucks, you'll probably want to use a flared port (or two, depending on your subwoofer size and power). It is possible to achieve the same result from both subwoofer boxes, so the decision between flared ports or slot ports depends on your specific needs for your custom subwoofer box."