4G almost all of the time.
I keep mine on 4G almost all of the time. The only time I don't is if 4G stops working and that only happens when I'm at home - although for some reason 4G has been working much better at my home recently. It used to be that I could never get 4G at home, but now I can get it most of the time. Anyway, I bought this phone so that I could get 4G, so I'm not interested in operating it in 3G. 4G loads web pages substantially faster in most everything I do and it makes a huge difference on certain websites that I use that have large and/or complex graphics.
As far 4G battery life goes, I bought the extended battery a few weeks ago and that made a big difference. So far, I have been able to make it through the day without hooking up to the charger. At first I didn't like the additional size and weight the extended battery added, but within a few hours I became accustomed to it and now it feels just fine. I can actually get a better grip on this beast with the extended battery.