Personally I think some of the perceived improvement of Keane is just down to us looking generally better as a unit. If you look at him statistically last year, he was still pretty good at winning headers, blocking, clearances etc, he was just shown to be very very limited when isolated. Last year he found him being isolated quite a lot, and he was often playing with Jagielka or Williams, who couldn't help him out when it happened. This year he's been isolated less, and Zouma has shown himself to be very good at getting back round to cover when necessary. That's not supposed to be a criticism of Keane, in a way it's a defence of him, in that some of what happened last year was outside of his control. In my opinion though, he's still more limited than I would like in the long run.
Zouma, on the other hand, is exactly the type of player I think we need at this stage of our development. He'll make mistakes and have poor games, but if we could get him on a permanent deal I think he could be a real asset to us for years.