On pogotoolkit.com you can get an estimate of the CP of your evolved Pokémon. In your case, these are the expected CP:
- Your Poliwhirl will evolve into a Poliwrath with 894-923 Combat Power (CP).
- Your Poliwag will evolve into a Poliwhirl with 483-486 Combat Power (CP). Your Poliwhirl can then be evolved into Poliwrath with 917-952 Combat Power (CP).
Since the difference between the two Poliwraths is quite small, it is cheaper to power up your Poliwhirl due to evolution costs (unless you have a lot of Poliwag candies and don't have much stardust). As dly mentions, it is probably better to wait until you have enough candies to evolve it or wait for a Poliwrath.