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.
I lived in Calcutta for many years, and even though it is more compact than Delhi, I think it takes a longer trip to really soak in the experience. It's not a place to go to for just sightseeing. Also, having taken many friends through Delhi for a couple of days, I would say that you should do Delhi for the 2 days. The immediate impact will be more memorable, because you can probably see 6-7 of the big sights easily: Qutab Minar, Red Fort, Humayun's Tomb, Lotus Temple, Parliament, Rashtrapati Bhavan etc.
Calcutta is for those who want the human experience of India. Delhi is better for a dip just to see the sights.