pokemon-go
Let's assume I have 2 Pokemon of the same kind with identical movesets and the only difference is that one has an IV of 100 and the other one has 90.
If I were to upgrade both of their CP to 3000 would they end up "equally strong" or would the 100 IV one be stronger?
While Kaz Wolfe's answer may have been true at one point, it is no longer the case. While with friends today, we found a Bayleef. All three of us caught the Bayleef, but each Bayleef had different appraisal results.
Thus, not only were the Bayleef's levels different, they had much different IVs, as well.
To see which one is objectively better, use the Silph Road research page.
Two Pokemon with the same CP tend to perform roughly the same, ignoring movesets and typing. However, Pokemon with exceptional values in some stats may outperform other species. For example, a Snorlax will typically last longer than a Raticate due to its very high HP and Defense.
I did a calculation via the Silph Road IV Rater to show this:
A level 13.5 Raticate with CP 500 (roughly in the middle) can have between 53 and 61 HP.
A level 6.0 Snorlax with CP 500 (on the high end) can have between 102 and 107 HP.
IVs aside, this low level Snorlax will most definitely tank more hits than the Raticate with equal CP. In the case where both know Hyper Beam, though, they are equally valid choices, since with STAB, Hyper Beam is the most powerful move in the game.
For another example, I'll compare Lapras and Cloyster, both Ice/Water types.
A level 9.5 Cloyster with 500 CP (roughly mid-range) can have between 41 and 47 HP.
A level 6.5 Lapras with 500 CP (roughly mid-range) can have between 87 and 92 HP.
Given Lapras' inherent tankiness, Lapras is a better choice than Cloyster.
If basing your decision HP alone doesn't settle it for you, look at the species' base stats on the link I provided above.
Best Answer
I will (slightly) disagree with Markol's answer here. CP is a computed term based on the IVs (Attack, Defense, and Stamina). It attempts to show how strong a Pokémon is based on those IVs, but it is biased towards Attack. (See PokeAssistant's page on CP for the formula details.)
In some cases, overweighting Attack is appropriate - particularly in PVE where you don't care nearly as much how quickly your Pokémon takes damage, you're really in a rush against time to deal damage.
However, in PvP, Attack and Defense are both valuable (equally so, for the most part), as well as Stamina. As such, CP doesn't perfectly reflect the usefulness of a Pokémon. In PvP, two identical Pokémon CP-wise but not IV wise, the higher IV one may be worse, or may be better, depending on the details of the difference. Two that are the same level but one is higher ATK and one is higher DEF, for example, the higher DEF one is likely better overall for PvP but the higher ATK one is possibly better in PvE (or identical).
Additionally, in PvP the ATK value will determine tiebreakers (currently) for charge move priority - a higher ATK Pokémon will go first, in a mirror match, if both fire off their charge move on the same turn. (In a non-mirror match, it's unlikely the base stats would be sufficiently similar for IVs to matter.)
All that aside, two points: