Poster, because although it maybe fragile you can always frame it. The risk is the same when you're transporting it regardless. What you do to prevent that is the difference. So I choose to have amazing visual quality for better enjoyment.
I always prefer wall-scrolls, easier to move around, and it doesn't miss to many fine details. A poster I would always frame, every poster I've had is otherwise way too hard to put up, and looks a bit bad (needs to be flattened)
Having two beautiful wall scrolls in my room (one I got for Christmas, the other I got from a Chinese restaurant lol), I don't think a wall scroll would take away from your art quality, if it's your art
i think your art deserves the high definition of a poster, and it's not hard to get a frame to hang it, which will also protect the poster. 20x30 isn't all that big to be honest, it's big but not unmanageable. still, if the scroll looked nice enough i would probably buy that on a whim over a poster just cuz of the novelty.
Poster! Unless you have a need to move and transport it or you like rearranging your posters/room often, I imagine the risk of damage is minimal. I mean.......it's going to be on a wall, isn't it? * V * );;