Should I upgrade?

I would say go for it if you can afford to. I upgraded to the 6 Plus from the iPhone 5 and am very happy with it (love the bigger screen and battery in particular). I had my iPhone 5 since its launch day and just upgraded 2 weeks ago.

 

Does your iphone 5 qualify for the free battery replacement program?
Check eligibility here:
https://www.apple.com/support/iphone5-battery/

 

If it helps you to make a decision any, my iPhone 5 is brand new. It was a replacement about a month ago.

Even so, there's no way I'd ever get 2 days on standby and I still charge my phone every night. About one day of use is all I expect and that's with a brand new phone.

My previous i5 had it's battery bulging, which is why it was replaced. I get better battery life now then I did then and even then, despite the bulging, the batter was IMO still good (charge-wise).

My point is that your battery life doesn't seem abnormal to me. But my previous phone was on iOS 6.1 and the replacement is on 7.1.2. So, I can't really say how much charge I'd get on iOS 8.

 

Also remember that all batteries start to degrade after two years. If you're otherwise happy with your phone you may want to look into the 3rd party battery replacement services.

I did that with my iPhone 3G before its 2nd birthday and the difference was like night and day. You get used to it and forget just how much worse it gets over time.