My phone was shutting down anywhere between 10-20% but mine was doing it on iOS6 and also on iOS7 so I took it to genius bar and they hooked up phone and it showed needing repair of battery, tech said it had to be severe to show on their system, bought my iphone 5 new it was 1 yr 4 months old. So I had them install new battery. It's been a week and no more shutdown's and battery is giving me 7-8 hrs of usage now (after I did a full restore of iOS 7.1) and goes down to 1% every time now. I'll buy iphone 6 but only if it's bigger screen and I won't buy first month usually wait 3 or 4 months in.
My 4s only exhibited this after the iOS 7 update and still happening under 7.1. My phone is way past warranty, and like you I'll just wait to see if the iPhone 6 offers a bigger screen. If Apple surprisingly doesn't offer a bigger screen this fall, I'm going to go Android.