They are really essentially the same thing, so price may rule the day. I have used both. But as southlake points out, the concentrates have the advantage of providing you easily scalable water to juice ratios. If the can instructions say 3 cans = 1 gallon, you might start out pumping up flavor by going 4 cans = 1 gallon, then scale up or down from there depending on results.
Concentrate is also an excellent way to backflavor and sweeten wine prior to bottling. Some people reduce the concentrate even further on the stove before use, but I just dump in thawed cans of it until the flavor I am after is achieved. It rarely takes more than 1 or 2 to a five-gallon batch on the back end.
Concentrate is also a great way to goof around with final wine flavor. Since you are already past the preservation stage when you are backflavoring and backsweetening, you can add, for example, a fruit punch concentrate to a concord wine, even if the concentrate says it has preservatives in it. Won't matter.
Juice has the advantage of being storable without refrigeration for some period of time before use. Concentrate is more compact to store but requires a freezer.