I think one thing that does need to be said is that the modern colonial empires of the twentieth century were generally more constrained by international norms than Rome. The Dutch, for example, showed extreme brutality in their attempts to suppress Indonesia, and the scars from the British response to the Mau Mau uprising are deep and long lasting, but they didn't exactly raze cities and enslave population. One must also note the rise of nationalism as an important factor.

But more importantly was the style of imperialsm was very different. It was a very real possibility for a Roman provincial to become a part of the Roman imperial system--Claudius, for example, appointed Gauls to the Senate, and Herodes Atticus, a Greek, became consul in Rome. A larger issue is that the Romans did not exert nearly the sort of social and economic control over its subjects that the colonial empires did. The colonial empires utterly reoriented the economies of their territories, as can be seen in the "deindustrialization" of India, the creation of African mono-economies, or the Indonesian plantation economies. While the Roman empire certainly changed its subject's economies, it wasn't in the same way. Ephesian an Syrian craft production, for example, was famous throughout the empire, and it is quite easy to see the wealthy and influential Tyrian and African trading houses in the port of Puteoli--and you don;t exactly see Nigerian trading houses in London.

So I would say the answer is that the Roman Empire was both more brutal and more inclusive than the colonial empires. It wielded larger carrots and sticks.