You can tweak the levels much more finely with a gate valve than with a riser tube. Both methods work, just the gate valve allows for finer tuning.
A good gate valve isn't cheap either. I want to say mine was $10-12 for the size I used on my ASM. It does allow me to tune it just the way I want it though. It has tight threads on it and takes a lot of turns to open/close, so the adjustments made can be very small and precise.