1.BSNL is good in Kolkata(Ranikuthi Exchange), not a single major problem till date (except a week long disconnection when the Reliance Idiots cut the line while digging theirs-may be intentionally).
Speed-4.25/5(1.5-1.8Mbps in NightTime, got 90GB in one month!!)

Customer support-2/5(C'mon, you guys know better than I do)

Cost-4.75/5(Very Cheap for Home users, night time speed is good.)
2.Airtel-Good customer service, but expensive.
Speed-3.5/5(Home user plans)

Customer Support-5/5(One word-Fantabulous)

Cost-2.5/5(Too costly to afford for Home users, nigh time speeds not good)
Total- BSNL-11/15

Airtel-11/15

So it's your choice, Speed and Cost vs Reliebility and Customer Support.
P.S-This is strictly my own opinion, please feel free to post your owns.

