Difference between Skimmer and Rocketeer?

It's different as they have different paths of missiles following behind the big bullet. Also, Rocketeer is much smaller and easier to be damaged

According to the wiki:

Skimmer

A similar design to the Hunter but with wider Cannons, this tank has a Deployer and shoots a Destroyer Bullet with two Small Cannons attached at its back flanks. However, the main Bullet’s damage is significantly decreased. These bullets are known as missiles. As of press (Keyboard) time, only this tank and the Rocketeer use missiles.

Rocketeer

The Rocketeer fires bullets with a Machine Gun Barrel on it's back, propelling the bullet forward. These too are missiles, as above.