Definitely the iPhone 4! My father had the HTC on android and didn't really get on with it for a range of reasons he has recently swapped it for an iPhone.
There are so many useful free apps for the iPhone on the app store. The ones I use in particular are AeroWeather, NOTAMs, Logten Pro, Air Nav, Memory map to mention but a few. All are really easy to use and being an apple product it has the intuitive user interface you become accustomed to!
I'm sure both phones have their pros and cons but for me the iPhone wins hands down.