The Mecha Wojira paid DLC gives you a mecha worm that won't lose health if it doesn't keep eating creatures, plus it has an infinite Death Beam, homing missiles, instant space jump and a special theme song. Basically you get an über mode for $0.99.
The main difference between the regular and Santa versions is that instead of being a giant worm who eats meatwads in order to grow and be able to spit bolts of death, you eat elfs in a Christmas environment and fight Santa. And one is free right now, while the other is $0.99. Just look at it as one being a Christmas themed visual makeover for the other.
Whichever version you play, the Super Mega Worm games are fun pixel art arcade games that are small enough to keep on your iDevice at all times. Go check it out!