The Legacy model MAY have a back-drivable gearbox (a good thing!), and a slightly slower gear ratio. The gearbox is cast into the chassis, and the wheels and axles are captive, so these aren't easy things to change. Electronics can always be upgraded, but in stock form the Legacy decoder has 128 speed steps vs 32 for the older model. The feedback encoder for Legacy speed control is a more reliable optical design that also allows for a larger flywheel.
They're both nice models but the Legacy products have some improvements that are reflected by the higher price.