Airtel has so much restrictions on FUP that its impossible for real Downloading. That's why many people go to retention plans which comes in all variety. The retention process is actually becoming quite a trend in areas where Airtel has competition. But Airtel won't offer you much if your area doesn't have alternate connectivity options.
If you wanna go on MTNL, check out condition of your local Infra - EDP panels, wiring etc. Because most of the MTNL Infra is rotten & bound to give you problems. Many MTNL plans are also unreliable like Freedom 800 Plan where users don't get speed boost at night. New 8 Mbps plans where users are not getting adequate speeds.
MTNL 3G Jadoo can be good if you have MTNL tower nearby. It has FUP of 500 GB & speed is 2-3 Mbps. Cost is 1600 Rs.
And if money is not a problem for you, then you should take Reliance as a secondary connection with 2 Mbps/4 Mbps Plans without FUP which cost around 750/- to 1200/-
Keep Airtel for reliability & Reliance for heavy downloading. (That is if you are heavy downloader for more than 200 GB per month)
Or simply take 8 Mbps Airtel-UL-Rapid Plus 2299 80GB, 4 Mbps Airtel-UL-Turbo Plus 2099 80GB Plans if you are not a big downloader.