Whoever the best driver for Red Bull would be, depends in my opinion on their future car speed in relation to the other teams. But does Red Bull want the best driver or the best _fitting_ driver next to Vettel? That’s a question nobody of us can answer.
Will RB still have one of the fastest cars? Maybe Ricciardo would be the better choice, IF Vettel can manage to get enough points for the constructor’s championship by himself. It would be a shame for Red Bull, if a team like Mercedes was even stronger next year. With Hamilton and Rosberg, they have two strong drivers, who could take at least one championship from RB. Slight advantage to Ricciardo, but with some risk.
What if RB loses the top spot next year? In that case, Räikkönen would be the best team-mate next to Vettel! First off, if the championship is off the cards, it won’t matter if Kimi will gain more points than Vettel or not. Secondly, the Fin’s good race craft and his constant finishing in the points make him an attractive choice to, for example, place the team in second place instead of third or third instead of fourth. Definite advantage Räikkönen.
Will other things play a role? Räikkönen will definitely earn more than the Australian Ricciardo, but it’s only natural to pay a world champion more than a young driver, who gets the chance of his life to drive in the one team, which has won so much in the past few years. Then again, according to news articles Räikkönen has a guaranteed salary of 3 million Euro with Lotus this year, but the contract is laden with performance incentives. I don’t see an advantage for any driver in this.
Daniel Ricciardo is no Vettel. In my opinion, he hasn’t shown extraordinary performances in his career at Toro Rosso and he hasn’t been the most consistent driver. Why should Red Bull choose him, when they could take Räikkönen for two or three years and then look to the next generation of young drivers out of their own program?
In the end, Ricciardo’s slightly higher experience (and arguably equal performance to Vergne) shouldn’t be reason enough to have him drive a RB.