Obviously they are completely different players. Both with huge upsides but in limited roles. Ibaka has played in 49 games. While Harden has played in all 58 games.
Some basic stats.
Ibaka 5.4pts, 4.9rebs and 1.1blks...16.6minutes
Harden 9.8pts, 3.3rebs and 2asts...23.1minutes
I think Ibaka has a decent advantage defensively. Harden's "D" has improved through out the season though. And Harden easily has the offensive edge here without a doubt. However Serge is 1 for 1 from 3 point land.

Some other stats.
Ibaka has had 8 or more rebounds 12 times.
Harden has score 14 or more points 13 times.
Ibaka has had 2 blocks or more 19 times.
Harden has had 3 assists or more 18 times.
I think what Serge brings to the table is the biggest need for the Thunder. Right now Harden is the better player.
But who makes the bigger impact?
If you had to hand out an award for the best rookie on the Thunder who would it be?