More efficient horizontal movement?

I'll start working in a horizontal tower defense for mobile and I was wondering what would be the most efficient way to handle the units movement. The movement of the unit's it completely horizontal. Something like this:

<img src="http://gamesaku.com/wp-content/uploads/2013/01/Battle-Cats-Screenshot-1-1.jpg" border="0" />

Which mechanic is more resource friendly? Platform, bullet, or just working directly with something like: this.x + this.x * dt

I believe in all the behaviors, but, to make this kinda of game I usually use the platform behavior when the terrain is irregular and the custom movement when not.

That's simple to use the platform behavior, bullet and sine behaviors, but I prefer the custom movement because it will use all their variables, while the platform will need a better AI.

so, if your terrain is in line, go ahead and use the bullet behavior. Set the direction of movement (your sprite always start moving of left to right) and make the conditionals meet the collision events to change their sprites for a fight animations, while setup the speed to 0, easy like this!

The terrain will be completely plain, so I think I'll go with bullet behaviour. Thanks!

skyhunter93 don't miss to give away from the gravity (it should be = 0)..