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."
Dreadlash doesn't suck at all. Revenge works very well with the nature f the vortex bonus, and his ability when active is very powerful.
He's the kind of card that makes Opp's wary of spending too many pillz. Couple him with pill wasters like deea or dagg and you can manipulate the match well.