I think in today's NBA you keep Ibaka over Harden 10 times out of 10. Harden is a good talent, but you can pick up free agents to cover up what you would lose if he walked... Ibaka was the first guy in 3 years to average over 3 Blocks per game (Which is a shame to see, just goes to show how wing-orientated the game has become) and has a much higher upside than Harden imo.
Although if Harden leaves they would definitely need to replace his scoring hole. That's one of their biggest weaknesses - outside of Westbrook, Harden and Durant they have 0 scorers. That would be even worse if Harden walked.
TBH I see them keeping both though.