If you're only hauling hay, get a deckover flat bed and use straps. Don't even bother with tarps. Tarps are for hay storage, not transport. Get the biggest trailer your truck will handle. If you have a 1 ton truck, I'd say get a gooseneck.
Sides and an enclosed trailer will cost more, and they only get in the way. If you don't use 1 ton bales yet, soon enough you will. You won't want sides.
If you have anything else you plan to use this trailer for, that changes everything. Now you have to start thinking about deck height, dovetail, etc.
I agree
Only I prefer a gooseneck no matter what size truck or trailer