It should make no difference at all.
Each Pokemon you obtain will have two sets of values and a level, which are all used to determine the CP value. All Pokemon of the same species will share the same base stats, while each individual Pokemon will also have a unique set of individual values (IVs).
If you look at Pokemon at the same level, you'll find that the possible CP values are dependant on the IVs of each Pokemon. That is, at each level, there's a range of possible CPs for each type of Pokemon.
A level 1 Bulbasaur can have a CP value between 11 and 15, depending on the IVs of the Bulbasaur.
If you power up a Pokemon, its level will increase by one half level. The now level 1.5 Bulbasaur, will have a CP value between 24 and 31.
If you had two identical Bulbasaurs (not likely), you would find that either way you chose to power it up, you'd end up with the exact same Pokemon.
Keep in mind that you can only ever power up a Pokemon, to 1.5 levels above your trainer level. So for a level 1 trainer, you can only power up a Pokemon to level 2.5.
For the following example, the trainer level is 1, and we have two of the worst level 1 Bulbasaurs ever (0% IV). So in both scenarios, the Bulbasaur starts out at CP 11.
In the first example, we'll power him up to level 2.5 (the max we can). At this point, we'll have a Bulbasaur with a CP of 49. Once the Bulbasaur is at max level, we'll evolve it all the way up. After the first evolution, we'll have a 79 CP Ivysaur. Evolving again leaves us with a 131 CP Venusaur.
Starting with the second Bulbasaur, We'll evolve it into a 18 CP Ivysaur, and further evolve it into a 31 CP Venusaur. After powering up the Venusaur to level 2.5, we'll be left once again with a 131 CP Venusaur.
As you can see, in both scenarios we ended up with the same exact Pokemon.
Keep in mind, however, that as your trainer level increase, you'll catch more powerful Pokemon. So if you evolve a Pokemon when you're at lower trainer levels, you could find that you're catching more powerful wild Pokemon later.
All that being said, there's at least one advantage to evolving first. And that's knowing the evolved Pokemon's move set, prior to spending large amounts of stardust to power it up. If you evolve a Pokemon and the final evolution has a bad move set, you might not want to spend stardust powering it up. If instead you powered it up before evolving, you'll be stuck with a high CP Pokemon with cruddy moves.
Data courtesy of The Silph Road