HDD For sure.

You will also want

1. Make you store them in an environmentally controlled space, no hot Damp Closets or Attics
2. Add some static Protection (either in an anti-Static bag, or if you are able to get a Case Pack for Hard Drives they normally have anti-static foam and are great for Cold Storage)
3. Add in some Desiccant packs as well

For me Personally I also mirror the data on my cold storage, 2 drives with the exact same data, I also go an extra Step of Adding in 10-20% Parity on each drive using MultiPar to protect against failed sectors

I also boot my cold storage every so often (try at-least every 2 years) to verify the data