Two main factors that your decision may hinge on:

1. Flying experience to India - With Skyteam, you can connect in either New York or ATL. With Star Alliance, you will be connecting through Frankfurt. Do you have a strong preference towards a 3-4 hour flight followed by a very long flight, or would you prefer two relatively long flights with a layover and gate change in between? If you anticipate that your trips to Mumbai are going to continue long into the future, then this would probably weigh heavy in your decision.

2. Use of your accumulated miles - With Skyteam, your flights out of Denver will have to connect in one of the following cities: Salt Lake City, ATL, Cincinnati, Houston, Cleveland, Newark, Minneapolis/St. Paul, Detroit, or Memphis. But with Star Alliance, you can fly direct from Denver on United to most of their destinations. So when you cash in your miles for a dream getaway, will you be okay with having a connection or two, or do you really want a non-stop? This should also be a consideration for side trips that you make. (Of course, if you don't mind the extra connections on Skyteam, your side trips will earn you more miles.) Where will that dream vacation likely take you? While both alliances have pretty similar coverage to Asia, Latin America, and Europe, Skyteam is weak in Australia. Skyteam has more comprehensive service to some of the smaller islands in the Caribbean, however.

I think that by balancing these two factors you should probably be able to make a decision that works best for your concerns.