I just tried the DX version. The AI is way better than in the normal version. Worth the purchase, even if you already have the normsa version. Newcomers should buy the DX version, and only the DX version...
I do think they should have just upgraded the normal app instead of making a new DX version. It's a bit of a rip-off for the people that already bought the normal version when it came out. Like me...
I'm disappointed that you have to buy a new version, and if the only improvements were sound effects then I would have passed - but an improved AI is very tempting.