I have better battery on 8.1 and the phone is a little quicker and smoother as well.
Just make sure you reset the phone (go into setting, about and reset) before you do the update and then after the update as well. Don't restore from back up either, just go to the store and download all the apps fresh.
Your getting a new phone so you will not only have worse battery for the first few days because of 8.1 but also because the phone is new. Be prepared to have bad battery for at least a couple of days.