The S4 isn't enough to squeeze the equivalent of an extra 800mAh out of the battery, especially when it's powering a larger, 720p display. It might do half of that if everything else in the phone runs efficiently. So you'll definitely take a bit a of a dip in battery life. Not a huge one though.
As for never getting the S3, that's fine. It's like I've been telling people in the 2 or 3 PMs I get each day asking me which is better between the MAXX and the S3. I tell them all the same thing - both are excellent devices, but which is better entirely depends on what you need and want out of a phone. The MAXX is the sturdy, tried and true, all-around reliable sedan while the S3 is the flashier, slightly more cutting edge new sports car.
You definitely seem like the sedan type.