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).

So if you want to choose between the two, I'd pick the CP 281 Poliwag. It will most likely cost you less stardust and candy to get it to maximum CP.

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.