Kernel Up-gradation to 4.9
Aeromental.atx
Comparing 3.18 to 4.9
"Linux kernel 4.9 or 4.4, which are far more secure and performant than Linux 3.18 was"
“If you rely on 3.18y, you should upgrade. But really, go use 4.4.y or 4.9.y, those are both much better and far more secure,” writes Kroah-Hartman.
So basically a upgrade. More support than 3.18 which reached it's EOL (End of Life, means it won't be maintained anymore with the upstreams you see) & 4.9 is in Mainline which will receive futuristic updates & upgrades.
Though don't expect a boom! in regards to performance slash battery, you're owning a device which can't even get a working video recording with GCAM 6.
Hardware has it's own limitations, but yeah! Greater Stability is what everybody should focus on.
TL;DR: 4.9 is Upgrade over 3.18, more stable & is being maintained.