That is the question. Evolution has good points and bad points. You just have to look at the situation from all angles, and decide from there.

Now, let's take a look at the good side of evolution. First off, evolved Pokémon generally have better stats. The Pokémon Lileep has base stats of HP:66, Attack:41, Defense:77, Speed:23, Special Attack:61, and Special Defense:87. Its evolved form, Cradily, has base stats of HP:86, Attack:81, Defense:97, Speed:43, Special Attack:81, and Special Defense:107. So, as you can see, the evolved form usually has boosted stats.

Another good side is apperance, although this is a lesser aspect. Most of the Pokémon look a lot cooler or tougher after evolution, but this would be a bad point if you like those cute, adorable Pokémon.

A disadvantage of evolution is moves. Evolved Pokémon learn moves slower than their pre-evolutions. For example, Marill learns Hydro Pump at level 45, while Azumarill learns it at 57. Some Pokémon (especially those that evolve with a stone) won't learn those moves at all.

So it's really your choice. Do you want better moves faster, or better stats faster? Or do you just want to go for appearance? It's all up to you.