I'm afraid what you're asking is like what's better, being a Democrat or Republican?! Whichever a person is (or uses, in this case) is going to get the rave.
I know NOTHING about Droid, but I love my iPhone 4. I would never use anything but Apple. I have drunk the KoolAid. I won't switch to Verizon until my contract with AT&T runs out, so I'm not buying a new phone (the 4S), but I am pretty sure you'll love the iPhone if you get it. I have many friends who have the 4s and I've never heard a problem about battery life.
I can't do a comparison for you since I have never even touched a Droid. Sorry.
Good luck!