Two twenties, because after a few months, I have two 15s and you have a twenty. Batteries never seem to fully charge like the used to.
My wife has a iPhone (the 4s or something) and still has to charge hers more than I do (Samsung Galaxy II). Not a whole lot different, but instead of her being tied to the outlet, I can pull my battery out, put in the new one and keep going.
That kind of convenience is worth everything to me. Also 200 dollars to replace a battery that lasts a year is still a lot less than I spend. I spend 20 bucks for two spare batteries on Amazon. I don't go through 20 batteries a year, that's for sure.