I've not sailed the Legend but it's a Spirit class ship. I've sailed the Pride, a sister ship of the Legend 3 times and have sailed the Dream back in Feb.
The dream is quite a bit larger than the legend--
Dream-- 130,000.00 tons--length-1004 ft-width-122ft-passengers-3646
Legend---88,500 tons-length-963 ft-width-106 ft.-passengers-2124.
Legend was built in 02--Dream less than a year old.
The Dream has a laser show on deck at night, a mini golf course, basketball ct.,a 303 ft long water slide and another one that's spiral and 104 ft long.
Basically it has more bells and whistles than the Legend.
Personally, if given the choice and the itinerary's were about the same, I would choose the Dream because it's so much newer and larger, has more to do, especially for kids.
Either would be a great cruise-- but as I said, personally for my choice, it would be the Dream--but--that's just me.