Well I'm owning a DJM 2000 by myself. And I "was" quite happy with it.
Later after buying the DJM 2000 I got some chance to play with the DJM 900 Nexus. And say what, just since i played with the 900, I am always thinking of selling the 2000 to get the Nexus.
The Effects on the 900 are just more intuitive than the DJM 2000's. For me the Nexus gives me much more prossibilities and Style in combining effects. And you have Post Faders, Post Fader Effects in the Color Effects Section, more Effects on the Beat FX Section, like the Spiral or the Melodic (great Effects!) And the Touch Strip on the 900 is just great to get instant, good sounding Effects.
Well with the 2000 it feels like you are much slower to get the Effects working like you want to hear them.
And the touchscreen Effects are mostly useless.
But the best way to find out what fits best for you, go in your local Music shop and test both of them!
But in my Opinion I would choose the 900 Nexus. :/