Anyone know what he plays on? Are they nylon strings, by chance?
I'm not a guitar expert but I believe Marcus uses a Martin & Co acoustic guitar very similar to the Flat Top Style 28, he sometimes uses a sound hole pickup to use with an amp. Judging by the crisp sharp sound and the fact that he fair dinkum slams that guitar (in a good way) I'd say the strings had to be steel. Its true that often people of the Folk genre play nylon for the advantages of plucking the strings, but I think he'd just kill it. As I say I'm not an expert so don't quote me on that, but I do know if you want one you'd better start savin' your pennies.
Sounds great though doesn't it.
I love Mumford & Sons
I think he uses a Martin D-28. That's as much as I know. Here's a link to that guitar: http://www.martinguitar.com/guitars/choosing/guitars.php?p=m&m=D-28
He plays a Martin & Co. D-28 .And yes - It's definitely steel strings.