data push is like a constant refresh of the data for whatever app youre using it for (email?)
fetch only checks on the interval set - uses less data (and less battery) by not constantly checking it but will only deliver emails on the interval
got an important email you need to get - set to push
otherwise, save the battery by using fetch
Uhg! This statement is incredibly incorrect. The exact opposite is actually true.
Your Droid X uses the C2DM framework from google for pushmail. As do all 2.1 Android OS devices. It's a gazillion times more battery efficient then fetching email.
Here goes one of the more complicated explanations out there.
Start with blocking out all thought of data transmissions and just think about your plain old cell phone. It registers itself onto different towers as you drive about the city. As your signal strength dips it queries the tower for information on adjacent towers and one tower hands off your phone to the next. The towers knows who you are, it knows close to where you are. At any one moment you're signal is being processed by 3 different towers.
Now, lets say you set your phone to battery saver mode and after 15 mins the 3g radio is shut off. Does this mean you won't get push email? Nope, you'll still get push mail because push doesn't depend on the 3g radio. Instead an "intent broadcast" transmission is sent via the cell network and tells your phone to wake up the application and even load the application if it's not loaded. It will then accept the incoming message through 3g.
A Fetch, on the other hand, completely resyncs the mailboxes and this takes significant time and a decent amount of data being sent over the 3g radio. If there was no new emails that entire sync was completely pointless.