Dark Pulse hits harder with Nasty Plot than with Solar Power. The only downside to Nasty Plot Houndoom, is that Houndoom is frail and it won't be easy to set up a Nasty Plot. While with Solar Power + Sun, you don't need to set up, and get almost as much power, along with a stronger Fire Blast and ability to use a devastating Solar Beam at the cost of losing some health each turn.Which is why you use Nasty Plot instead.
Having used both, as long as your enemy doesn't have Drizzle or Sandstream Pokemons, its easier to use Solar Power Houndoom. + The Sun is useful for weakening Water moves and I like being able to use Shiftry, who is very fun to use.