Both are OK.
The first is more neutral, detached.
The 2nd shows perhaps what you're thinking right now, it's more implicated/involved. You're giving access to the listener to your inner thoughts at the time.
However, this is to over-analyze. Many people would use them alternately. http://thegrammarexchange.info...Type=1&search=Search
Both show up in published books:
20 on "I plan to study law"http://books.google.com/books?...22&btnG=Search+Books
4 on "I'm planning to study law"http://books.google.com/books?...22&btnG=Search+Books