MDO I think is better, it's designed for commercial use making signs so it's very smooth and easy to write on and even better to erase on when you make a mistake. And it's plywood designed for outdoor use. The trick for me is finding it.

MDF though is really nice to work with, it just doesn't like water.

If I have a choice I'll use MDO for the fuselage frame kayak frames, it's really nice to work with and takes oil well so it's easy to seal. Makes frames for stripping nice too.

Bill H.