I suspect Poecile will come along soon and put us all right, but before she does, I'll add my 2p's worth. I'd say Marsh.
I never go with the shiny cap or pale wing panal thing as these can vary according to lighting conditions or feather wear. The bib is a good clue, being smaller and neater on Marsh, but this varies with the age and sex of birds to the point where they can't be seperated on this IMO.
Willow Tits tend to have a larger rounded head which gives the bird the appearance of having a bull-neck. You can't see this well on your photo, but I do think this would be more obvious if it was a Willow. The cheeks are also whiter and extend further down the mantle on Willow, perhaps the most obvious feature lacking in your photo.
The sure fire way to tell them apart (except on call) in my opinion is the pale cutting edge to the mandible, which I've never yet seen a Willow Tit with(although Poecile may have to remind we whether Marsh's always have them). Unfortunately you can't really see the cutting-edges to the mandibles on your photo.