The energy math
A single-speed pump runs full-blast whenever it's on. A variable-speed pump runs slow and quiet for most of the day — using dramatically less electricity — and ramps up only when you vacuum or run features. Over a season that difference often covers the price gap.
Quieter and gentler
Lower speeds mean less noise and better filtration, because water moves through the filter more slowly and thoroughly. It's easier on your plumbing too.
What to look for
Match the pump's flow to your pool volume so you turn the water over about once a day, and confirm it fits both in-ground and above-ground setups if you might move it. A built-in filter basket is a nice bonus.