The single biggest difference between Tata Indicom Broadband & BSNL Dataone Broadband is the upload speed. With BSNL your upload speed is limited to 5-7 KBPS. With Tata Indicom Broadband / VSNL your upload speed is limited to 512 KBPS.

The other major difference is the availability of static IP with Tata Indicom Broadband. Unless you are planning to spam, having a static IP is a very big advantage for multiple reasons:
1. It allows you to host your own website from your computer
2. It allows you to secure your access to your internet servers by restricting access only from your static IP
3. It allows you to whitelist your IP, if necessary, from RBL's.
4. It allows you to establish your digital identity on the internet.

The biggest non-technical difference is that Tata Indicom has a polite and responsive customer support which actually solves your problem.
BSNL / Dataone Broadband has a place where you can file complaints, but no steps are ever taken to resolve your complaints, you will never get any customer support. Saying BSNL customer support sucks is being polite. BSNL customer support is non-existent.