Well I never had a droid so I'll speak mostly on the iphone 4's behave. iPhone- front view camera back view camera, light weight, tons of apps you can use for financial,business,pleasure,etc. Driod X- I think has a keyboard, blockbuster or netflix connections, yada yada.
Having almost no experience with Droid and a lot more experience with things like iPhones and such I would definitely say iPhone 4. But then again, that's my personal opinion and everybody has their own. What's good for me may not be good for you :)
The Iphone 4 is a better succesor then the Droid X