We've looked at both titles in the past here on All About Windows Phone, and it's tough to say which is better - they are targeted at very different styles of game play. But they are both very good at what they do, and trial versions are available for both, via the AAWP App Directory.
Monster Burner (review)
Your wizarding role is to defend the bottom of the screen from the approaching enemies and you do this by throwing fireballs. You're not actually represented on screen, your fireball will appear where you touch the screen, and a flick of your finger will set the direction of the fireball as it heads towards the enemies.
You can increase the capability of your fireballs through an in-game purchasing system, and these power-ups stay with you once purchased, and you will need this extra capability as you approach later levels. Bigger fireballs, more destruction, and the ability to bounce off the side walls will all help to get through each level with a high score while collecting as many coins as possible.
Fusion Sentient (review)
Let's put the stock science fiction story aside (rest assured, it does all build up to something that Michael Bay would film), what we have here is a classic tactical war game. It's level based, and at the end of each level your army of robots will have gained experience (making them stronger), pilfered weapons and technology (to make them more deadly), and you'll move on to the next part of the game (with an ever increasing challenge from the enemy). Move through the story, exploit the tactics learned on earlier levels, and do your duty to capture territory, enemy flags, and kill anything that moves that isn't yours.
I'm delighted to report that Fusion: Sentient has the balance of tactics, speed and UI just about right.