Motion Pro Delux mercury, but if I was buying one today I would probably try the Morgan. The rods can't spill out or get sucked into the engine as with the mercury so it looks more user friendly to me, especially when you need to snap the throttle a few times or rev it up. Also, the rods won't separate at their tops and make it hard to get a reading. The absolute cm Hg isn't usually very important anyway, the vacuums just need to be the same. If you are careful with the Motion Pro, you can avoid the two problems I mentioned above by careful storage, clamping off the tubes when you rev it, etc., but it is more of a hassle. Haven't heard any bad reviews on the Morgan.
