Both of these Pokemon can shine if put on the right team. Here are the pros and cons of these two Pokemon:
Omastar (Omastar Pokedex)
"Siiiigh" Well the biggest downfall on this guy is no Sturdy! In competitive battling this says you need to use a Focus Sash. He also has a poor 55 Base speed. This just screems, put him on a rain team! If he is on a rain team, use Swft Swim. If he is not use Weak Armor. He also has 4 weaknesses. 1 is 4x.
He has a great attack stat which makes him a deadly sweeper if he is able to set up. He als has a great movepool featuring plenty of Water and Rock moves with a sprinkling of Dark/Ground/Ice moves with a lot of great status moves such as Toxic, Toxic Spikes,and Iron Defense.
Crustle (Crustle Pokedex)
A pitiful 45 base speed. Only 3 weaknesses but only steel can be covered. Last, a weak attack stat of 95. Not many cons on this guy.
He has Sturdy!! "WhooWhoot" He has a huge movepool mostly rock moves but with a lot of other stuff in here. He has the basic bug moves and a few Ground and Dark moves. Some great moves such as Counter, Poison Jab, and Endure.
Overall, Crustle has less downfalls but Omastar has a great Sp.Atk stat making him a better sweeper if on an appropriate team. My opinion, take a look at his and decide.