I highly recommend SSD of HDD but if you are a newbie I don't think a migration would be easy at all.
I suggest preconfiguring your PC with SSD or you are ***.
how much battery life are we talking about? From my experience, the cpu is more important. So Samsung EVO 850 500gb m.2 SSD is my OS (for R2 & R3) drive while the 1tb is the storage drive. I usually torrent into the 1tb, download movies, upload movies into youtube.
My old returned 17 R2 has i7 4720HQ, usually last 5 hours for battery drain overnight test.
Current 17 R3 with i7 6700HQ, usually 8 hours for batter drain overnight.
For people with windows 10, i suggest disabling super-fetch and windows search (services.msc)
realistically at least 25% at the minimum
coming out of sleep makes the HDD at 100% for a while and when it does random virus checks it's at 100% so thats 25W every 100%
SSD makes it like 5% max because it does it so quick and the watts at 100% are like 5W