What kind of motor would be better for mapping?
There are several ways to do do this:


Use stepper motor (as Jodes suggested). Because the step angle of the stepper motor is given, you can measure the time for which the wheels kept turning and figure out the distance (Hint: The distance covered by one complete rotation of the wheel is equal to it's circumference).
Use an encoder (As John suggested). Lots of material is available online.
Use a GPS module. This might sound overkill but might actually be a real life saver depending upon the estimated area which you are looking forward to map. 


Both #1 and #2 are prone to slippage and other mapping issues because none of these techniques making your bot 'aware' of it's precise location. So, there is a possibility it can count the same distance more than once.

As for #3, you can decrease the on board complexity by your board simply acting as an explorer and transmitting it's co-ordinates to a pc via a wireless link or something familiar. The PC can then do all sorts of fancy things with the data (including drawing an actual map). 

But then again, the correct choice depends upon the use case scenario.