The adjustable cushioning is the main reason to get this one over the Z5.0. If you are in this price range, $800 should be well worth it. This is the treadmill design that put True on the map and the most quality compact unit you can buy!
True is the only company that has perfected the orthopedic belt system. When you combine this with their adjustable cushioning, it makes for an excellent treadmill experience. If you have the small space that needs a smallish treadmill, then the Z5.4 makes for a great experience. If you have any orthopedic problems, go ahead and get the Z5.4 model from this lineup. You will thank me since you can adjust the cushioning to perfection for your particular needs. The fact that it is whisper quiet is an added bonus.