There is a stigma attached to cleaned coins.
For common coins, a buyer wouldn't even look at the ones on the left. They might be more interested in the ones on the right but they would be able to tell that;
1. Old coins shouldn't be that bright
2. Coins that look uncirculated shouldn't have so many signs of wear and environmental damage on them.
3. the coins were not in their 'original' condition
If they were rare coins and there weren't many others to chose from, then most likely the ones on the left would do better unless you can say they were 'professionally restored' to get them to the ones on the right. In which case you would probably be better off letting them tone naturally so they coins didn't look like they had just come out of the dishwasher.
Many Roman coins have been dug up and cleaned and had corroded parts taken off of them to prevent further deterioration. I have several hammered coins from England which have been in a field for 100s of years. They are all cleaned to remove the dirt but none of them have been taken back to a bright shine and the guy who sold them to me said he wouldn't let me have any more if I was threatening to polish them. I have polished a few and they just look wrong! having a 300 year old coin that is bent and worn but looks like it has just popped out of the mint is very jarring to the eye. Also a bit of dirt in the creases helps the worn design stand out better on the coin's background.
Ultimately a collector should chose to walk away from both coins and keep looking for a decent quality coin.
However many coin collectors I know also clean dirt off their coins and are known to give it a bit of a scrub as well. Most stop short of polishing them but most feel a bit guilty about any sort of cleaning. It doesn't stop them though.
In the order of acceptability you have;
1. picking a bit of dried dirt off with your thumbnail
2. running the coin under the tap
3. running it under the tap with a bit of soap
4. Soaking in acetone for silver or olive oil for copper
5. Soaking and then giving it a light roll with a cotton tip
6. running it under the tap using your thumb or a toothbrush to dislodge harder dirt.
7. soaking in cloudy ammonia to remove grease and dirt stuck in the grease
8. polishing with bi-carb of soda
9. dropping in coke
10. scrubbing with steel wool or a wire brush
Most people have their limits.
I have personally cleaned many kilos of coins that generally wouldn't be considered to be worth collecting. I buy them at just above scrap value and spent a ton on cleaning them.
