I'd say maple. Sycamore are more of a v shape (as are the seeds) and they have a more serrated edge. But of course, there's always a chance this is a slightly "smoother" than normal sycamore, so don't count on me!
From the look of the bark in the first picture, I'd say that's definitely a field maple. Sycamore bark is darker and doesn't have that ridged 'corky' appearance. Also, if you can find any young twigs, if it's a sycamore the stems of the leaves will be bright pink.
I found one of these 'copters' in my field the other day & started to have a panic - through much googling etc I'm certain it's either a Norway or Field Maple, therefore not poisonous. Can you see the seeds? As if they're 'U' shaped they're Sycamore but straight like the one in my photo are ok.