Check your current mobile plan with your carrier. While the iPhone 4 is available from Verizon and AT&T, the HTC EVO 4G is only sold through Sprint. If you're unable to jump between carriers, your phone selection will be limited.
Try out each phone. The iPhone 4 features Apple's iOS operating system, while the EVO 4G is based on Android. Both operating systems are based around a similar icon-heavy user interface, but differ in areas ranging from keyboards to file browsing. By trying out the operating system on each phone, you can determine how comfortable you feel doing tasks such as navigation and web browsing.
Consider your potential smartphone budget. As of the date of publication, the iPhone 4 comes in 16 and 32GB models, priced at around $200 and $300, respectively. In contrast, the EVO 4G is priced at about $100 through Sprint.
Check the app selection on each platform. Thanks to increased cross-platform development, most popular apps are available on both Android and iOS phones. However, if you need a specific mobile app for something such as work, double-check the developer's website for platform availability.