Simply between the two cities, Calcutta is far more compact and in two days you can comfortably get a feel of the place. As well, not so much for the sights but for the urban experience, I think Calcutta wins hand down. It's a place you can walk, while a walk in Delhi/New Delhi always gets tiring after a half-hour or so.
Of course Delhi wins hands down for grand historical sights, but you simply don't have the time to do them justice. Best to wait till your next trip when you do.
You don't mention when you're going, but I wouldn't suggest either city in the heat or rains.