You mean to say that the speed depends upon the proximity of the nearest BSNL tower, and I guess it's the same case with airtel and tata indicom too?
I have no experience with Airtel and TATA Indicom but I've heard that this BSNL EVDO thing is not at all stable. I mean for browsing this is the best option as of now. All of my friends who use BSNL EVDO say that the speed varies a lot and the speed depends upon the time of the day and various factors. Airtel is GSM. So that Airtel data card is useless. It'll give at the max 150 Kbps which, according to me, is bullshit. TATA Indicom provides Photon service (similar to BSNL EVDO) but I guess there are no unlimited plans and area covered is very less. Also, compared to TATA Indicom Plug2Surf, BSNL EVDO is obviously much better.
Conclusion: If you're in desperate need of a wireless solution to access the internet then BSNL EVDO is your best option. Note that ping from BSNL EVDO to any server in this world is usually ~1000 ms but I guess that's normal considering that it's a wireless service.
And please, don't even think of Reliance. I hate Reliance - so does everyone.