You have to decide what you truly need the trailer for, do you just need something to haul around mowers and other equipment? Do you have a dump truck or other large vehicle to carry debris, mulch, other material? If you have that go enclosed.
But if you need this trailer to haul mowers, debris, and easily be able to access and load everything you have? Than you would get an open.
You also have to decide this, do you feel like unloading equipment every time it rains or to avoid threat of stolen equipment? Can't beat being able to leave your equipment in one spot all year round.
I bought an 14' open trailer, and am happy I did, you'd be surprised how quick your truck can fill and being able to pack the trailer to can turn 3 trips into 1, can't really do that with an enclosed.