I like the Sentinel series. It is good, solid tower defense. The graphics are great, and the upgrades for towers are pretty hefty. It gets challenging toward the end of a group of 10 levels when you have a boss enter the arena.
Legendary Wars is more castle defense. It has gotten good reviews on the app store, but castle defense is not my thing, so I passed on it.
Personal preference should probably dictate what you buy. Personally, I'd go for Sentinel 3, but that's just me. They are both relatively inexpensive; maybe you should just get both!