I do not think you can go wrong with the X. Personally, I prefer it over all of the rest. However, I have not had time to play around with the DP.
The X gives you nice versatility from size to performance without any major drawbacks. Not that the rest aren't versatile but they do not offer the size with the exception of the Samsung BUT a MAJOR drawback for me was the lack of LED notification. Another strong point for the X is its build quality. You never get the feeling it was made cheaply. Same goes for the D2.
The X really is a great device that packs some unique options and if you do another 1 year upgrade, it will be enough device until 2012 which, is when I think the true LTE evolution era will begin. LTE hand held devices are not something I would recommend this early (although they will be cool!) because you will pay more monthly, have limited LTE coverage for the $$$, lack of software development (will catch up eventually).
So, go ahead make the plunge on the device you are comfortable with and 2012 will be here before you know it!