You’re weighing speed & energy usage vs cost. With a small array, HDDs power draw difference probably won’t amount to much and you may hit the RPi’s I/O bottleneck before fully benefiting from SSDs speeds, though latency would be better.

But TL;DR: If your goal is to learn and build something cheap, go HDD. If you hate it, you can upgrade over time.

Oh, sorry. They meant, “No smoke; without fire” it’s like the firewatcher’s “All clear”. There’s no smoke and they’re without fire.