I'm sure you'll get a few responses to this from different points of view. Here's mine.
The XD9 has GREATLY improved the drums and some sounds but the overall effect will be just a slight improvement because the X1 already sounded so good. The advances that I liked were in the button layout, although some things were better on the X1. This will all be determined by your comfort level and if you need certain fast access to things. For me - the XD9 has an advantage in the rhythms section for sure, and the solo sounds are tweaked up a little too. The biggest improvement is the acoustic piano sound. If that's important, it may be worth the upgrade for that alone.