US Gear is my choice. I have used it with two coaches. One gas and one diesel. It works equally well with both. It uses full vacuum assisted braking, is fully proportional with breakaway and system monitoring in the coach a standard feature. The systems that tap into the air brake systems such as M&G and SMI on diesel coaches are frequently not installed properly. Most installer just tap into the rear brake air. If they are installed with out the appropriate protective air system parts being installed on the coach chassis they can cause big problems if there is a fault. The US gear system is totally non-invasive and my choice for either gas or diesel. Properly installed it works with the chassis engine brake to provide barking to the towed vehicle as required. To my knowledge, none of the air controlled systems are capable of this.
Al