Nylon looks better longer than leather, is quiet, light, and easy to clean if need be. If you buy quality products, it looks professional. If you buy cheap crap, you'll look like a soup sandwich. And the gear won't work well.
Leather is noisy, especially at first. It requires maintenance, and blemishes are hard to hide. It's heavier than nylon, which adds up after 8, 10, or 12 hours. It's appearance changes with age, whereas nylon doesn't. It's also more expensive.
Here's what I use on a daily basis for patrol.
I wear a 2 inch nylon belt (I've used all the major brands; they all seem to be about the same to me), a Safariland 6004 holster body for my HK45/Surefire X300 combination that is mounted to a Quick Locking System (the holster body is finished in Safariland's STX coating...it's very Kydex-like). The Quick Locking System is great; I can switch the holster body from a duty belt to a thigh plate in 5 seconds. My personal radiation detector carrier is DeSantis nylon, double open top magazine carrier is from Raven Concealment Systems (the same one they sell for concealment, but on 2 inch belt loops), 2 sets of ASP handcuffs (chain and hinge) in ASP nylon cases (do not wear them in the small of your back!), nylon radio carrier is Uncle Mike's, my Surefire flashlight lives in the molded plastic carrier from Blackhawk. Until I switched to the RCS mag pouch I used a Center Mass integrated magazine pouch for a 20rd PMAG for my M4 mated to a Safariland pistol mag pouch, but now I just stow a 30rd PMAG in my thigh pocket. My ASP rides in a hard scabbard. I also use a cell phone holder from Maxpedition. The rest of my need-to-have gear is in my BDU pockets. Anything else is in a bag in the car or the trunk. BUG holster is Desantis as well.
Be aware that some nylon products look like shit. Pick the good ones. I'm obviously not devoted to any one brand; some of the stuff was issued to me (Uncle Mike's) and I kept it because it worked fine. The rest I picked out using function as a guide. In terms of duty holsters, Safariland can't be beat.
With all that said, nothing beats leather for appearance, which is why we are mandated to maintain a leather rig for our dress uniform. My current preference is a Safariland river belt, holster, mag pouch, and a single cuff case (we're not allowed to wear anything more with our Class As).
In the end, it's personal preference. My only firm recommendation is that if you pick leather, give Safariland a long, hard look. They make solid gear.