

I think 3.5" are usually priced better per tb than 2.5" drives and performance is usually better too. So unless you feel like burning money for an inferior solution, are have some space constraints that doesn’t allow 3.5" drives, I wouldn’t go with 2.5" drives. They’re more energy efficient though, but you’d need a fuckton of drives for that to make a worthwhile difference in your power bill.
2 x 8tb HDDs is roughly $200USD
I don’t know what kind of electricity prices you’re paying, but to hit a 3 year ROI on your SSDs, you’re paying at least $2.2USD/kWh, assuming the full 15W (232kWh/year total) consumption of the HDDs and assuming negligible power consumption from the SSDs.
Edit: average power consumption for HDDs read/write operation is usually around what you claim them to idle at, with actual idle consumption below.
Edit2: and to be fair I did take refurb HDD price. a refurb SSD is around $300 USD for 8tb, bringing the minimum power cost per kWh down to ~$1.7USD/kWh for a 3 year ROI.