This is a good question. I actually had a very similar situation when choosing to continue saving candy for a Nidoran or to go ahead and evolve the Nidorina.
CP is directly correlated and determined by the individual values, or IVs, of your specific Pokemon. To figure out what those IVs are, you have a few options to figure this one out.
You could use the appraisal feature built into the game as of a previous update, roughly a month ago. This will give you a general idea of how high your Pokemon's IVs are. I generally use this to weed out objectively weak Pokemon before using the other method.
You can get the exact IVs of your specific Pokemon by using an IV calculator, such as the ones found here or here.
Once you know how your Pokemon stack up IV-wise, you have to decide whether it is worth training a lower-CP Pokemon with near-perfect IVs or going along with a decently-high-CP Pokemon with lower IVs. In the end, the higher IV Pokemon will have higher CP, but it will take more time to get there.