As a woodworker, I can say that MDF is far from the lightest material. In fact, it's often the heaviest. Plywood, OSB will be lighter. Particle board will be lighter too.
MDF and particle board don't like to get wet. So you'll need to consider keeping them dry, sealing the edges, etc. Neither take screws well. But MDF is flat and smooth and makes a great substrate material.
And, MDF, doesn't bend at all. It'll just snap off a big chunk. So depending on the support and the span, you might want to take that into consideration.
So far the woodworking I'm doing in ours is using 3/4 and 1/2" plywood. If it needs to look nice, then I use "Baltic Birch" plywood; if it doesn't then it's plywood from Home Depot.
When it comes to strength and stiffness, things get interesting. Again it depends on the span, but a sheet of 3/4" plywood might be strong enough but it may bend.
Glue two sheets together and you might get some stiffness back, but it'll weigh more and be thicker.
In an attempt to make this reply actually useful, I'd suggest starting with a sheet of plywood. If it bends too much, then we can try some things to make it stiffer.
One of the best approaches is called a "torsion box" where you sandwich a frame of solid wood supports between two sheets of plywood.
I'm just finishing up a knockdown bed for our RV and will post some pictures in a few days.