Generally combo but that depends on the combo....
I personally use 3 rare 3 star ninjas since they make an awesome combo, but if it comes down to similar powered common ninja? Stars matter a bit.
But if there is a common ninja good enough to use, u'd probably farm it to 5 star, hinata, kimmimaro are some of the best examples, they are good and thus most player have them at 5 star.