Hoondoom definitely .
Flareon is the weakest Eevee evolution so dont chose Flareon. Hoondoom have a greater movepool. Overall Houndoom is much more useful, but it depends on your team. Flareon is too slow and weak defensively to outclass much of anything. If you are considering using Flareon as a physical sweeper, just use Arcanine instead. Arcanine has access to a powerful priority move, and has much better all around stats.
Houndoom does pretty well in UU as a special sweeper because of nasty plot and good SpA/Speed. The Dark type helps with coverage and at least gives him an immunity to switch into. However, I have found that Blaziken can be more effective since it has the ability to be a mixed sweeper.