The Sport band is obviously very flexible and perfect for fitness. The Sport band does restrict your wrist from breathing. Also well made and durable.
The Nylon band is lightweight, extremely durable, and very comfortable. Apple's nylon bands are very well made. I have the pearl and black nylon. And the Nylon band will allow your wrist to breathe.
I find the Sports Band the most comfortable all around. The nylon to me just doesn't work. I think the strap is too narrow for the Watch and just can't get a comfortable fit one hole is too tight and the next down is too loose.
i have both the sport bands and the nylon bands. I only wear the sports bands when working out or when working in the yard or doing housework. They are easy to rinse off/clean.
I worry about the nylon ones getting dirty and nasty..]
I don't have a problem with the sport bands not breathing.
I liked the nylon when I had it but it started fraying within 2 weeks so I returned it. It was bit disappointing so I purchased Moko Milanese for $6 for breathability.