One of the best 0 cost cards.

Agree on this card, definitely one of the best cards out there for the cost. Re-roll a die and take another action.

Being able to give you the option of rerolling an opponents die cannot be underestimated, particularly if that die has a special ability showing. Keeping this card in your hand for late game is very useful for keeping your characters alive long enough to get some damage off.

Ambush =OP 0 cost => More OP Reroll a die => supra OP

My friend and I are debating this card vs Doubt if you have to choose one. Any opinions here?

Doubt provides removal and is a great target for Vibroknife, Overkill, Lure of Power, Holocron, etc. Unpredictable can help fix a die and resolve it before your opponent can answer with their own mitigation, or potentially mitigate an opponent's die.

I'd say run Unpredictable in your yellow deck unless you need an answer to vibroknife.

Doubt is currently a very good control card against some common dice in the meta (especially dice with lots of modified sides or pay sides if you can catch them when they have no resources). Meanwhile, Unpredictable is very flexible. You can obviously use it to potentially improve one or your own rolls rather than trying to worsen an opponent's. So it's not as strong for control because they can reroll into something as good or even better and it never removes the die so it can be rerolled or manipulated at any rate. Which you use would therefore strongly depend on the deck you're playing and the role you hope the card will play in your overall strategy. 

Generally, I think I'd include Unpredictable in more decks, but Doubt would always be my choice in a more control-oriented villain deck.