Absolute encoder or Incremental encoder?
You are probably better off with an incremental encoder. 

An absolute encoder will give you the exact position within the encoder's range of travel. For example, a standard single-turn potentiometer could be considered to be an absolute position encoder. You can determine the exact position of the shaft even after power up / power down. 

In other words, an absolute position encoder will tell you where you are within the range of travel at any time. If you should remove power, then reapply power, the encoder will still tell you what position the encoder is at. 

An incremental encoder simply provides pulses as the encoder is moved, along with direction information. 

In general, if the encoder has to move past its full range of motion, an incremental encoder makes more sense. 

I'll give a couple of examples. 

If your robot has a rotating platform (a head), an absolute position encoder is a good choice. You can read the rotation position of the platform at any time and the reported position will always be accurate. 

However, if you are measuring how far the robot has moved by measuring how many times the motor shaft has turned, an incremental encoder is more appropriate. 

Specifically, if the encoder shaft has to rotate more than a single turn, an incremental encoder does not provide  any benefit. You still have to keep track of how many times the shaft has turned.