Similar: Rome and Constantinople were the capital of the Roman Empire (at different times) Both had the usual Roman facilities, a palace, forum, etc Both were incredibly wealthy and populated 1million and 300,000 Both lasted a long time 600 years or so and 1000 years. Both had powerful armies, and at times navies.
Different: Constantinople was much more a Greek and Middle-Eastern influenced city Rome had pagan beginnings, Constantinople was always Christian. Rome had experienced democracy, Constantinople was always ruled by an Emperor (who was also - more or less - head of the Church) The Catholic church in Rome always claimed that the church that developed in Constantinople was not the 'true' church, and vice versa (Roman Catholics and Orthodox Catholics). Just put it like a pro and con shart if you tryna make this short.
Look up Rome and Constantinople in Wikipaedia for more information.