A few for you:
Cost: Even with the $10 surcharge for 4G data, Sprint plans will still be significantly cheaper than ATT.
Unlimited Data: ATT has discontinued unlimited data. Compare the plans and your data usage to see if this helps or hurts you.
Your tech savvy level: Android is a fast moving beast. The app market is not monitored like the Apple App store. If you're cool with, or indeed like the fact that Android 2.1 is already great and that EVO will get 2.2 (Froyo) in a couple months and that applications may be seriously flaky, requiring extra review time or just experimenting, go Android.
If you like to have things "handled" or sanitzed for you so they "just work" go iPhone.
EVO is a beast. Beware it's battery usage but it's an amazing phone and I can't wait to have one and turn it into a 4G wireless hotspot in my house! (Now I've blown my cover, yes, I'm an android fan.)
All that said, I'm pushing my mom to get an iPhone. She doesn't need to be confused by stuff and will freak out if presented with too many options. Apple will hold her hand and make her do it their way. Easy. ;-)