Are these garage doors? In any case, it depends on your skills and what you want the finished doors to look like. A few years ago I needed to build a 42"x6ft gate so I decided to make it with four panels like a door. The frame I made from mahogany and the panels I made from sign board. When I asked my lumber guy about how good the sign board was, he said, "It's just as good as your paint job." If you use the lumber, you could make it into a v groved ship lapped type of panel. With the plywood, you don't have to do that, but I would use a good grade of marine plywood. You might use sign board like I did. It's cheap and lighter than marine grade MDF and probably about the same weight of plywood. Your job will stand or fall by the jointery of the frame.

If you make it out of solid wood planks then you have to leave room for expansion and contraction within the frame. Plywood makes for a simpler, more forgiving construction.

What finish will be used?

If the doors will be painted it is not as important, but if you are using any finish that allows the grain to show mass production plywood is rotary cut and stands out like a sore thumb beside real wood.

Thanks so much for your responses. They are very helpful.

The solid wood will need considerable allowance for expansion in the design phase. Count on probably 3/8" of slack per foot of width (across the grain). If the panels are tight you have a catastrophe in your hands as the doors blow apart at the seams. Fix those panels with moldings so they could be removed and trimmed down.
Instead of regular plywood, I would recommend a medium-density-overlay (MDO) plywood, which is what sign-painters use; it is fir plywood with a resin-impregnated layer of paper on the exterior side. It is extremely paintable with no visible grain, and lasts forever out in the weather.
Casey