This is the SAME decision I was making when I started 40k. I went with Nurgle CSM because I love the idea of rotting space marines that wield plague magic and have zombie cohorts.
I bet Nurgle marines would enjoy Carcass or cannibal corpse.
I on the other hand, prefer my death metal a little more melodic, e.g. dark tranquillity, (old) in flames, arch enemy, or Therion.
That's something to think about. GK ally naturally with any space marine army, Astra Militarum, and Inquisition. Chaos tends to like... chaos, so Daemons and CSM ally well.Vote for GK here. I've spent probably less than half as much on my GK than on my old Ork army and it's twice the points value without a doubt. That's not great on the table, but it's great on the wallet.
Locally, at least, there have always been a bajillion Nurgle players while there have only been a couple of GK players even when the 5th edition book hit so I like that side of playing a niche army. That said, if you do a Nurgle army really well, you'll still be in a minority group of players if you want that kind of thing.
GK also have a way better Allies Matrix; in case you ever want to branch into another army but want to do it gradually while still playing games with your growing army, or you think your Codex is missing that one thing that really scratches your gaming itch, GK have a lot more options for that kind of thing than Chaos.
Indeed, there is more than enough variety within Chaos to keep you entertained. Chaos is basically 5 aesthetics in one (4 gods and undivided) on top of all the legions out there.Chaos also has a large depth to its collection though, with the number of different unit types (and the whole Trator Guard via IA13..)
But Iam Biased.
GK does have the whole Armys of the Imperiumness so they can include Kights and the like...
But they do have issues finding narrative reasons for some games though.