MTS license has been cancelled by the Supreme Court in the 3G case. Not a good option to become a new customer at this stage and pay for the device.
Also keep in mind that any wireless connection would not be as stable as a wireline connection and signal strength would vary.
While choosing between various wireless options first consider where you would be using the dongle most. There are primarily two technologies. CDMA-EVDO working on 800 Mhz (Reliance netconnect+, Tata photon, MTS, BSNL-EVDO) and GSM 3G working on 2100 Mhz (Airtel, Vodaphone, Idea, Aircel, MTNL, BSNL etc). Due to the nature of airwaves, inside house, CDMA gives better average signal strength than GSM 3G even though at bursts GSM 3G peak speeds are higher. GSM 3G devices are SIM based and you can use unlocked devices also, CDMA are mostly locked and without SIM.
BSNL 3G provides the cheapest possible option with zero roaming charges throughout India. However, at many places BSNL is known to have signal issues. So better check exact position in your location.