Susannah,
Depends on the budget. Solid-state hard drives are becoming less extravagent. They are much faster, run cooler and last longer on a battery charge. I have friends with those for primary laptop drives who swear they get six hours per battery charge.
I trade on a laptop, so running cool is as important as speed. On a laptop, the two are related. If too hot, the processor slows down. I may try out one of those drives. They are basically large RAM chips shaped to fit into the laptop's HDD bay.