Every time I use my friend's DroidX I'm amazed at how awesome that huge screen is. I'd say the performance of the three devices is similar enough that it won't make much difference, and the only thing I can say against the X is not being able to use custom ROMs. That said the new MotoBlur is very unobtrusive and looks nice overall, and the custom widgets are nice if you want to use them.
As I've said before, I love my Nexus One, but unless later builds alleviated the touchscreen issues, I have to give a strong warning that it can be very annoying, even if it rarely happens. Another common issue is it not being able to hold onto a wifi connection, though there may be some custom ROMs or kernels that have addressed this. Also of note is that you can no longer buy the phone from Google, so unless you can find a reseller with some stock, you have to be a registered developer to get one, and it'll be T-Mobile only and ship with 2.1.
I've only very briefly played with an iPhone 4, but I can say that it was a good bit heavier than I expected, and the build quality seemed to be top notch, as one would expect.