As is the case with most Gameloft games, very little is known about these two games short of the contents of their trailers. Thankfully, there is quite a bit of gameplay shown in both of them so it's not very hard to get a feel for each game.
The Settlers - The last in a long line of Settlers games starting with the original that was released on the Amiga all the way back in 1993. The Settlers is a real time strategy simulation game that historically has had a fairly relaxed gameplay pace which should translate well to the iPhone. The amount of units moving on screen at once is fairly impressive, and with how few of the games there are of this genre on the App Store, The Settlers should easily stand out from the pack.
Castle Frenzy - Castle defense games have seen a remarkable amount of success on the App Store, from the numerous games with simple stick figures to highly detailed games like Knights Onrush that all share the same basic gameplay elements. Castle Frenzy seems to have the same array of defense upgrades and abilities seen in most similar games, with the most notable difference being its colorful 3D graphics. It's hard to say what if any effect this will have on gameplay, but from the trailer alone Castle Frenzy seems to be one of the coolest looking castle defense games if nothing else.