I've been to Rome throughout the year, and have enjoyed it each and every time. It's a different experience at different times of the year, and it would be hard to compare one against the others.

My personal choice would be not to go to Rome in August if it's your only time visiting there, and without the oppurtunity to return for a second trip.
It depends, I guess, on whether you like to have the 'local' experience - eating where the locals do, and attempting the language - or a more touristy experience which it would be in August.