Share 'Shimano m520 vs m540 pedals'

I'm making a switch over to shimano pedals this year for cross and I'm trying to see if there is any difference in these two models.  I'm most concerned with their ability to clear mud and the easy of getting in and out.  I like that the 520 has pedal flats but I'm willing to forgo that if the 540 is a lot better in the mud.   Thanks Eric