As far as I know the whole GIF legal system has been resolved after their rights expired a few years ago.

MNG's are probably larger in size than GIFs because GIFs have an optimization format that means frames that share the same data(pixels) do not add to the filesize. So a head with 200 frames and only the eyes blinking will still be very small due to the optimization, whereas in MNG each frame will be stored like a separate PNG and be much larger in filesize.

I'd avoid MNG's due to the lack of widespread support. Stick to GIF unless you absolutely need to use MNG(for example, because you want alpha-transparency animations, but even then you can't use GIF2MNG).