You should go to a store where you can test out phones and see what phones are available for your current budget.
The one that feels best should be the one you buy. All batteries get worse. The more you use them the worse they get, that's a fact.
If you're like me and you're going to spend the whole day playing on it then the battery will get worse much faster. In about 9-15 months you'll already start seeing the difference.