The main advantage to an array is the ability to control directivity, plus you can get high sensitivity from a relatively compact box. The disadvantage is the high cost of using a lot of drivers, though that can be gotten around if you find a decent closeout deal. A horn has the advantage of being able to get high sensitivity from a single driver but construction is far more complicated and the box can be quite large.
Which to go with depends on your goals and how you want to best achieve them. My current HT setup has a folded horn sub and array satellites, each chosen to take advantage of the particular advantages of each design.