They will have different kinds of issues, but I wouldn't say one is necessarily better or worse. A stored trailer may have bad
tires, rust, mold, wood rot, critter infestation, and/or exterior deterioration. A well-used trailer may have scuffs, chips, worn upholstery and flooring, dirty appliances, and other casualties of use.
How much varies widely, and I wouldn't rule out either. Used often or not at all, some people take care of their stuff and others don't. Only a thorough, first-hand inspection will reveal the kinds and extent of issues you might have to deal with.