I have the X and my G/F has the Iphone so i have had some time to play around with both. The iphone is simpler to use and there is less stuff you can mess up, but with that the X has much more capability (but you can screw stuff up). For most people either one will be fine. If your a tekkie geek definately go with the X, if you are tekkie illiterate, Go with the Iphone.
The two biggest differences I have found with them are as follows.
The iphone Definately has a superior camera. I think it may be the software portion that makes it good. I can recreate a picture of about the same quality on my X, but it takes an aftermarket app (vignette) and alot of messing around with settings based on what light conditions i'm in etc etc etc. (while i'm messing around, my g/f is running around snapping pictures of everything with her iphone on auto waiting for me.)
but one thing I can do that she cant is USB tethering for free. I downloaded PDAnet, plug my X into the laptop and i'm away surfing the internet with my x as the source while she is surfing on her Iphone. Overall i'm extremely happy with the X and would get it again over the Iphone, but she would probably get the Iphone over the X. So really it's just personal preferences. other than those 2, there was no major dealbreakers either way for me. they can do pretty much the same thing overall. if one does not do it out of the box, you can get an app for it.