The two biggest differences are:
-Idle time basically being a standstill for iOS
-Apple optimizing for usage on web browsing. Whenever Anandtech does battery tests, iPhones kick ass on those two tests (wifi + LTE).
Bad coverage will always ruin battery life - especially for a battery that's probably through 300 cycles. Also, you're using a three-year-old phone. What did you expect?
Also, honestly, if you're having radio issues of any sort, I would recommend taking it to an Apple store. Even out of warranty, they've given me a new device more than once.