Tell them to send you an MMS message outlining the reasons. Oops! That's right - they can't.
So you send them the reasons you like your BB and have them copy and paste it into another email. Oops! Sorry, they can't do that either.
Have them compare their spare battery to yours and see whose.... Oops! Sorry, they can't do removable batteries, only fugly adapters to expand their phones.
PS: the iPhone won't sync GMail any better than the Blackberry. It's GMail's fault anyway.
PSS: If the app store is the best reason iPhone users can give you for why it's a better device, have pity on them. They have no clue what a real email communications device is all about.