The Mammut Pro Cord is our favorite cord for making a cordelette. It is strong, durable and easy to untie. While it is more expensive than normal accessory cord, it is not THAT much more. We think the extra cost is well worth it. This is the stuff you see many guides using for setting up anchors and works great for setting up top rope anchors. The only downside is that it is a little more bulky than accessory cord. If you use the 6mm diameter size (our favorite), then you definitely have a little bulk on the side of your harness. Overall, it is the best size and weight to get the job done in most climbing situations. For big wall climbing, the fact that it comes undone so easily is awesome. You won't be spending five minutes trying to undo the master point.
One minor dislike: we do wish the pre-cut lengths were a little longer for the 6mm size. It comes in a 5.5 m length (18 ft) and we wish it came in a 20 ft piece. But that is a minor dislike.
The pro cord is made of nylon. Go to our climbing sling review to read about the difference between nylon and Spectra and Dyneema.
You can also buy this cord in the following lengths 2 mm, 3 mm, 4 mm 5mm, 5.5mm 6 mm, 7 mm. We like the 6mm length the best because of its strength, durability and ease of untieing. If you are alpine climbing or trad climbing, you might like the 5 mm or 5.5 mm size. However, for big wall climbing, we find it is harder to undo the 5mm size after weighting it.