The gameplay is the same. You're allowed to travel to areas you've visited from the start. And you only have two party members, the third members are monsters you fought in battles. But I think you'll like it.
As for the DLC, well... it's a big maybe. You have bonus bosses. Some you can beat as early as chapter 3 or 4, other require a full eveloped party. So if you're looking for a challenge, there you go. Also, every monster from the Arena can be tamed. The three bonus episodes... they depend, really. Sazh is an easier and more fun way to get casino coins. At least that's what I think of Chronibind. After you've beaten the episode, you'll receive Sazh as a Synergist-Monster. He's not bad, but I don't like to use Synergist-Monsters. Requiem lets you fight against two boss battles - it also contains major endgame spoilers. The DLC starts out to be hard, but with every fight, regardkless of winning or losing, Lightning will increase in Level, making her stronger, obviously. Winning both battles with a 5-Star-Rank will unlock Lightning as a Commander-Based mosnter. I consider her to be a game breaker. The Snow-episode is something I've never played, as I don't like Snow. Not only can you fight Snow at the Arena, there's also another boss added, which can drop unique monsters. Like Silver and Golden Chocobos. There's only one in every game and Valfodr increases the amount. Doesn't sound too bad.
The DLC-Weapons? No, they are not worth it. Some weapons do sound like they have good skills, but there are plenty of regular weapons, which are very good.
Costumes on the other hand... if you think it looks good or cool, why don't you buy it? Serah has an outfit you can download for free.