How to turn off 3G on Iphone 5 after update?
I don't there is anything much you can do now since this is a change to iOS. After the update the toggle is only between 3G and LTE and no more 2G and 3G toggle. Also, you won't be able to revert to iOS 7 for long if at all, so try that if you really must revert this functionality.



From an experience standpoint, I really don't think you will run out of battery if you're on 3G. I would advise you to not use mobile data while travelling around a lot because with frequent changing of signal towers could lead to draining of battery.

Although I've seen a method to edit the preference file after jailbreak to fix this issue but it never worked for me.