Unless it is a trademark I very much doubt any legal action is possible. There are over 200 extentions out there, imagine I register gerrit.ru to take just a random example and someone else registers gerrit.us ; unless I first register a trademark valid internationally, I cannot see how the other person could lose his domain in front of a court. I mean, there'd be no point in different extentions otherwise. Only in case of trademark there'd be an issue as far as I know (correct me if I'm wrong)
As for the better choice... If you have the cash, buy both. Otherwise it depends on the country. In some countries (Australia, UK, Israel, Turkey, New Zealand etc) it is a habit to have .co or .com in front of the country code and people would not be used to the domains without it. In other countries nobody would care about the third-level domains because they are used to second-level. In some other countries (China, India, France, Spain, Russia, ...) both are available and it gets more complex. Depends I guess when the both versions were launched and what people were used to first. In India, .in was launched much later than .co.in so people are still used to .co.in a lot, whereas for example in Spain and France .com.es and .com.fr exist but are not used frequently simply because people aren't used to that .com being there. Complex situation but in case of India I'd say both .co.in and .in would have the same value more or less as both extentions are used frequently.