The plus size will give you better battery life than the smaller size, because of the larger battery. But yes, both have better battery life than their predecessors. Lithium batteries tend to last only about two years before they start going downhill, and the iPhone 6 was released in 2014. So it’s about time to get a new phone, and as said by Apple, you will get at least one additional hour of battery life throughout your day when you get the plus size. In your case with an aging battery, you’ll most likely get significantly better battery life.