If your truck is plumbed for a air trailer already and you have the license to pull it and don't intend to pull with any other vehicle, air is the way to go. Cheaper to buy and maintain and will be a much heavier trailer if you ever need it. You can find 20 ton tags on air for virtually nothing, cheaper then an electric trailer with 3 7k axles and will cost about the same to maintain. It's a lot more to get the license to pull them here. Many companies I know are doing away with trailers altogether and going to 5 axle straight trucks to get away from the license requirements of air brake trailer.
Electric brakes are good for trailers around the 30,000lb mark and lower, people who use pickups, or both pickups and heavy trucks to pull the same trailer. Despite what some may say here, electric brake failures are very few and far between and are mostly due operator error. A big problem is the way people hook and unhook the electrical plugs. In all my years of pulling electric trailers i've never had brakes fail completely. The worst is you start to turn up the controller to get enough braking force, and then you know you need to climb under and adjust the brakes and good for a long time. The brake shoes however are much much smaller and won't go nearly as long between replacements.
Here if you're over 26,000lbs gross truck and trailer need annual inspection, regardless of electric or brake brakes wheels must be pulled every 2 years.
