I have cheap router with 32 Mb RAM and 4 Mb flash (TP-Link TL-WR841N v8), and I'm not sure if I should update from 15.05.1, due to raising memory and flash requirements: https://openwrt.org/supported_devices/432_warning. There's build for my router, with 3,474K sysupgrade image, so it's probably compatible.
15.05.1 is very stable, but I'm not sure about vulnerabilities it has, such as recent wifi vulnerabilities.