I'm using a pre and an ipad.
you'll miss synergy. It's limited on webos now as far as what accounts you can tie in, but it's more than ios. And synergy will open up to whoever wants to write for it with webos 2.0. I.e. If you are following other peoples calendars in your google calendar,u will only see yours in the stock calendar app.
you will miss how easy it is to close apps. You do have to keep your # of open apps down or else an app will crash wile your doing something once in a wile in ios. Just want to be able to swipe open apps away.
wile typing, I miss the equivalent of control c and v t copy and paste. You can double tap, or hold, or something to do that, but wile typing, I like that functionality in the keyboard. Might be different with the smaller screen of the iphone, but that's what I think about typing with an ipad.
it won't turn the letter u into you or r into area. If the auto correct gets everything right, I love it, but it often times screws everything up, making it worse that if it just used what I typed.
just some contrasts