Hello John
I get first dibs on a reply to a difficult question and would like to hear others opinions.
I presume you mean muffler pressure and not crankcase or a pressure bladder/pacifier/surgical tube which all have their place on combat or high rpm set ups.
Muffler pressure has good and bad points and varying results depending on the motor and tank type.
Have found it works well on the higher rpm stunt set ups running a constant 2 stroke as it helps consistency and probably allows a slightly bigger venturi then suction does.
Muffler pressure seems to it stop going rich up wind because of the tank vent pressurizing and enriching the engine . It also helps stop surging or leaning if you hand launch.
Suction works well with a small venturi and larger engines that are doing the 2/4 break thing as it seems to give a better switch between the 2/4 stroke mode.
My comments are based on what I have experienced but I am keen to hear more on this topic from those with more experience/knowledge.
Regards Gerald   

  I take it on a plane to plane basis. I test fly a new model to work on the engine run, If I think it wants pressure, I use it. If it behaves without it, I don't use it. Even if it's the same brand and size engine.
  Type at you later,
  Dan McEntee