I actually have used both. I have found pMachine to be very flexible and a pure PHP/mySQL enviroment. Very easy to plug an exisiting design into. But other than Moveable Types Pearl interface it is also as flexible. MT is very hard to install, until you have done it once and then it seems simple. (If that makes any sense?) There are also many other options as well. But they are all mostly the same. MT and pMachine have excellant support systems. Go to the sites and read the forums. If you are concerned with Standard complient sites then MT has more support in that area as well as an extensive plug-in community.
Pick one, learn it and stick with it is all I can actually recommend.
