My company builds mobile apps so I have multiple smartphones.
Either will work, but one thing to consider is upgrading. Apple has done a great job of maintaining backwards support for their old devices. Since they control the hardware, the new iPhones really aren't too much different than the previous ones. That means that accessories are more likely to be supported in the future as well as they leave room on the device for the OS to grow
Android is more progressive, they have a lot of cool new features that allow you to really customize your device. The 'widgets' allow apps to put a custom control on the main screen that allows you instant access to information that you are looking for. You can pull the device out, check the main screen and put it back without opening an app. With android being open source, the operating system will be updated more. The down side is there are so many phone manufacturers out there. Not all Android phones are going to be good, make sure you do your research.
Personally, I bought a BlackBerry Torch yesterday. It's a workhorse of the phones. I use it exclusively for business and in my opinion it does it the best. I don't use many apps, and there aren't even close to as many available. But Email and integration with buisness tools it performs above the other two platforms. That said, I think over the next 3 years, Apple and Android will patch the difference in security and BlackBerry will lose alot of ground. They are also notorious for not being able to be upgraded.