Evolving in PMD will do very little to your stats as far as I know, unlike main games where there is a larger increase in stats. If you evolve them, there are certain exclusive moves (like Blast Burn and Wing Attack for Charmander, and hydro Cannon for Blastoise) that can be learned, and an extra type (in Charmander's case). However, they level up and learn moves slower, and getting an extra type may mean more weaknesses.
So actually, there is no real reason to evolve in PMD other than for collecting. So, evolving or staying as your current form can both work out. (most people choose to evolve though.)
(Hey! I had a Charmander and Squirtle too! ^^)