Hi there,
I can't tell you about your chances as I have no familiarity with the Indian system and how U.S. schools treat it, but I did not end up taking up UCL's offer because of the cost for the MSc and the high likelihood for no funding for the MRes year. It seems like the consensus is that since you won't have any information from the MSc when applying to the U.S., it's pretty pointless to apply in the MSc year. If you attend the MRes year AND rank well in the MSc, I'm sure you have a chance at a school in the U.S. if you apply during the MRes.
Good luck!