Hi,
Check the views on sify broadband at this forum.
You also have an MTNL forum at the same site. As you can see, there are a lot more people disgusted with sify than MTNL.
Sify is quite unreliable most of the time because the last mile connectivity is provided to you by a local associate, and you have no guarantee of uptime or service. You can get lucky at times however, but if you want to play safe, stay away from sify.
Sify manages to give good speeds most of the time, but unfortunately all their unlimited plans are actually having transfer limits, and hold unlimited only at night. The comparible Night Unlimited plan from MTNL sounds much more appealing to me, as it offers higher speed at a lower cost.