There are two valve options available for Michael Rath Custom trombones, both Swiss designed: the René Hagmann valve and the Willson Rotax valve.
Both valves have their own distinct playing characteristics. The Hagmann is designed to cause as little disruption to the airflow as possible and feels very open in the valve register, even through the pedal register.
The Rotax valve is the best option if you prefer slightly more playing resistance, which can help with projection and focus.
However the Rotax valve is nowhere near as "stuffy" as a traditional rotary valve and the response on the valve is remarkably consistent when going from the open to the valve side of the trombone.