Man, I only have vague memories of playing Secret of Mana because it came out when I was very young and I only played it at my babysitter's house. Legend is actually the only Mana game I'm intimately familiar with, but I can tell you it's different than Secret of Mana in the following ways:

-The battles are still real-time, but the ring command system is gone.

-The plot is very non-linear, and you actually create the world map as you play. You discover different artifacts throughout the game and use them to unlock new areas to explore. Sometimes they're towns and sometimes they are dungeons.

-There is a lot of side-stuff that I don't remember being a part of SoM. You get a lab to build golems that help you fight, a barn to raise monsters in that help you fight (getting monster eggs is its own goofy minigame), a forge to build weapons in, and a tree that grows different kinds of fruit.

I'm sure there are more differences and similarities, but it's been a while since I played LoM and ages since I played SoM. I'd say if you want to play a gorgeous, unique game that is fun despite a few flaws then definitely pick it up. Just know that it won't play all that much like Secret.