iPhone Blog has put together a post by comparing each of the respective strengths in syncing to the Cloud for Gmail, MobileMe, and Exchange. It's a worthy read.
Here's the bottom line all along: If you're an Apple/Mac user, the iPhone is generally the best choice. Yahoo Mail, MobileMe, and Exchange are all pushed to you. If you're an Exchange user, the iPhone is your only choice as Android does not support Exchange and will have to rely on 3rd party apps to provide the solution. Also, there is seamless sync of all the contact, calendar, and event information that works well.
If you rely on Google for a lot of your mobile apps, the Android phone from T-Mobile is may work out better in the long run. Google apps just work. Period. And if you want nothing more than that, the G is likely the best solution. You don't have to worry about Yahoo, Exchange, or any other services traditionally available on the Internet. Google intends to make itself your best mobile friend.
Now, Google's online applications for the typical mobile user is pretty good and with G1, it gets even better especially with push mail. This is a feature that is currently not offered on the iPhone. I don't understand why mail can be pushed for Yahoo but not Gmail. Anyone know why?
Impact on users: It'll depend on if you're a Mac or Google person. If you're both, the iPhone is the best solution. I'm willing to bet push mail for Gmail should also come to the iPhone eventually. Google is not about to make its services on the iPhone second-class.
Source: The iPhone Blog