put it this way. the MP kills the MBP in terms of performance as it is a desktop workstation with 2x quad core server grade processors (in stock model), desktop fully buffered RAM, hard disk and graphics while the MBP is just a professional notebook with a mobile processor, RAM, hard disk and graphics. still the MBP would be faster than some deskops out there.
it depends on what computing you will be doing. if you tell us what applications you will be using we could decide if the MP or MBP is better for you. and if mobility isnt a concern the iMac would probably suffice as well.