This is a complex question. Rome and Greece were both great civilizations and they both lasted so long. So, it really depends what time period you are looking at. For this answer, I will compare Greece and Rome at their peaks.
The peak of Greek power was probably under Alexander the Great. Alexander conquered all of Greece, the Persians, and even made it out to India. He was an unstoppable force. He was also a great military strategist. He created new weaponry and improved on the Greek phalanx. Since Alexander went East, he never faced a consular Roman army.
The peak of the Roman Empire was probably under the emperor Hadrian, at least according to the great historian of Rome, Gibbons. The Romans had a huge army of nearly 4 million; the number cannot be certain (again Gibbon's estimate). They also had incredible experience and very advanced military techniques through centuries of warfare.
In terms of land, Rome had more. In term of infra-structure, Rome also wins. In terms of a battle, again I would give it to Rome. Greece never became a nation. They were individual city states. Alexander, of course, unified them, but it was only temporary. The force of a unified Rome, in my opinion, was more powerful.