I have used both but i fond BSNL to be much more reliable and stable as compared to Reliance broadband. Reliance broadband usually suffers from problem of high ping due to poor routing and they also throttle torrent downloads and for this particular reason i kicked them out of my home.
Also Reliance broadband is quite expensive is billing is concerned. They will charge you with massive Rs. 100 fine for late payment of bills and they also demand advance deposit for all their broadband plans because they fear that their customers will disconnect their broadband connection for good.