Rather late noticing this thread.
I have been using Airtel for last several years, having dumped BSNL because even they admitted that their cables in my area were old and useless. They have since recabled. but I've stuck with Airtel.
Airtel's call handling and admin side is useless. It is a vast faceless company. They will close calls just to get them off the open list. Against that, and more to the point, the guys on the ground are good, and if you have any problem that is not fixed quickly, you can their number and phone them direct. I suspect Airtel encourage this to keep their call stats looking better

!
The Beetel modems from Airtel are amazingly cheap (probably BSLN too). You can go and spend thousands in the open market, and have a much fancier box, but Rs.850 is pretty hard to beat for a working modem (wireless/switch is a one-thousand-something). Also, my Netgear modem/router will not keep the link up with our line conditions, but the Beetel modem seldom drops. I got the first Beetel when Airtel gave them out free, swapped in my own Netgear when it failed, but just recently it giving outage of several hours a day: fixed by buying an Airtel Beetel
I don't know if anything has changed with BSNL in recent years, but Airtel has a different culture, and its cheaper, because the engineers come, do the job, and go. There is no hanging around with open palm --- even if they have worked long and hard to fix a tough problem.
Download speeds may not be the immediate fault of the provider. Undersea cable faults, even though far off our shores, have taken a toll recently. I could not get any decent speed until last couple of days.
Also in the news not too long ago: major BSNL cable damage somewhere like Poonamalee High Road area (???).