The question is do you want the extra features and expandability of the X8500 or are you most interested in the appradio experience?
The X8500 and the appradio 3 (assuming you're getting the 210) both have DVD players, both have stereo bluetooth, both have touch screens, although the X8500 might not be capacitive. I did a google search and didn't find an answer. Both are fully appradio compatible with all compatible phones, also neither come with any cables, both are going to act the same in appradio apps, but the appradio 3 is 150 dollars cheaper.
Now, the X8500 does have an SD card reader, two USB ports, so you can plug in more than one device in at once and you can plug in USB sticks full of movies and music, it has HD radio, is satellite radio ready with an adaptor, it has one more preout, so if you're using external amps, you can control 4 separate channels and a separate sub, it supports auto equalization based on your vehicle(no clue how well that works), it comes with a remote control, and finally for theft protection the display is removable. <-- that's a lot of extra features for 150 extra bucks.
If those above extra features mean nothing to you save some money and get the appradio 3. Hell if you really want to save money, buy the 110 and ditch the disk drive. Who uses disks anyways? Diskless the difference of price is now 250 dollars. However, if you find the above features to be an awesome addition, and worth a 150-250 dollar price premium the X8500 is your radio.