Under most circumstances, Deea is superior due to an unconditional ability, better damage, and pill forcing.
I would only use Dreadlash in place of Deea in a deck with a strong confidence finisher (Naele is most obvious, Simeon and Stanly are also good options), or during a week where 9+ powered cards are rampant.
Consider this a fair warning, but I'm about to spoil the first half of the subject in under 600 keystrokes.
Me; "Run both."
You; "It's not always possible to run both. If you had to choose: Deea or Dreadlash."
Me; "Depends on rest of the deck and what the banlist looks like. If I had to choose: I'd go with the gut feeling."
You; "Yes, of course. I mean in general if you /had/ to choose on face value alone."
Some other dude who looks a lot like Slify; "Dreadlash but most people would choose Deea"
You post a preset.
Some fourth dude; "I prefer Izsobahd."