yeah, totally personal preference. i tend to unclip at weird angles going into a run, so 51's would be disaster for me.unlike digidelia I like tight tight tight cleats that just won't pop out until i tell them to...for CX I come off when i want to and not before...trying the unpossible climbs in CX and busting out unexpectedly is murder
I would move my used pedals off the mtb and toss on the CX, as slightly worn spuds actually hold better. then get new ones for the mtb. if going new go light that means shimano's best mtb pedal available
shimano m970 on CX rule, you may need to perfect the 'mud-slap' to clear out the cleat after running but these are pretty good at stepping into clogged.
I was an early adopter of the M-980 XTR pedals and while I liked them for cross when it was dry, but after wet/muddy days racing masters at the USGP in Maddison and Fort Collins where I had issues with lots of mud/muck buildup in the spaced in the pedal body so I opted to put the M-970s back on the cross bikes and the 980's went on the 29er SS.I run the M-780's (newest XT model). The new XTR's are more carved out, have a bigger contact area, etc, but so far I've had two sets of them break on me.