Gaara - People will only suggest the MCP350 to be a good pump if you plan on modding it which can be risky.
Other than that the main competition is between the 50z and MCP650
although that Maxxx guy said it's not, 50z seems to be the bandwagon everyones jumping on, but when I said that he said "no the MCP650 is the bandwagon', but I hear nothing but 50z from them. They said it's supposed to be quieter, perform better, and put less heat into the water.
The MCP650 is quiet, so they are crazy there, I have a friend who has it. About performance, I've only seen a couple charts in which the D4(MCP650) outperforms the 50z, so I asked maxxx if he had any reviews that proves all these claims of the 50z being superior....All I keep hearing is biased opinions so I'm still waiting on that.