I've used the MX20 extensively (300 hrs in our Cardinal) and a 296 briefly. The MX-20 is an amazing display. I love the symbology, especially tied into the SL30/GX60 we have along with it. It is truely integrated, and works beautifully. That said, I don't think it is worth the extra $3k unless you want:
Traffic display (pretty crowded on the 396 screen already, and avidynes new 600 series TCAS systems look tempting

).
Or if you plan on using ADS-B in the future (which I will be planning on).
We'll see when I get to that point in 3-5 years, but as of right now I'm planning on an airgizmos mount of a 396 instead of the MX20, but if I had more money, I'd spring for it. Of course, I'd rather have a GNS480/396 combo than a 430/MX20 combo.