I think it's worth it just for Order of Ecclesia. It's my favorite of the DS games. When my apartment got robbed, and I had to replace my handheld Castlevanias, it was the first one I went for. It is difficult at first, but you get used to the gameplay. Bosses are especially difficult until you learn how to fight them. I don't think you can get away with tanking out many if any of them.
Dawn of Sorrow is easier, but has a lot of cool features. For instance, the extra character mode has a story to it, and lets you switch between three characters on the go. It also plays a lot like Symphony of the Night, so if you liked that one, I'm pretty sure you'll enjoy this one. Lots of weapons and such, plus a cool magic system with the soul collection.