For a standard Les Paul, and most other carved-top guitars where the pickups are mounted by rings, either will work. Long legs are 'classic', but short legs allow you to raise the pickup higher more easily.
For most guitars with flat tops and the pickups mounted in a pickguard, use short legs. For direct-mount pickups, use long legs for carved tops with higher bridges, such as tune-o-matics and wraparounds, and short legs for flat tops with lower bridges.
It's generally best to go with short legs since they can be fit to any guitar, while long legs may not. There is never a time when you need long legs, unless you're trying to pass off a pickup as an authentic '50s unit.