There are numerous differences that generally favor the MK-V (non-Field version), and they are pretty well outlined in the QST product review on line here:
http://www.arrl.org/members-only/prodrev/pdf/pr0208.pdfI've used the 1000MP and the 1000MP-MK V, but not the Field, so I relied heavily on the product review noted above in drawing my conclusion that I'd prefer to have a used MK-V than a new Field -- unless, that is, I'd be operating "portable" (such as Field Day) a lot with it, in which case the Field's ability to work from a 12V battery renders it more useful.
WB2WIK/6