The advantage of a union ball valve is that it has a union at both ends so that you can remove it and clean it -- being a true union means that you can unscrew it and remove it from rigid plumbing without having to bend or flex anything.
Depending on what you're plumbing (a sump return line, for instance), you can often kill two birds with one stone, and have it do double-duty as a ball valve plus a convenient place to disconnect the plumbing for service. This also usually saves on space (vs. a ball valve plus a union or two) when working in tight spaces.
At the local big-box home improvement stores here, true union ball valves usually run $5 - $10 more than plain ball valves, depending on size. Worth every nickel the first time you need to remove it, usually.