They are both fine Both Bloomsbury (Russell Square Station) and South Kensington are on a direct tube line from Heathrow. From London City Airport you will need to take the Docklands Light Railway and connect into the Tube at an interchange station. Don't forget that the Tube makes up only a part of London's excellent transport system.
We have 500 bus routes Light Rail and Rail as well. Don't pay cash. Buy yourself an Oyster Card or Visitor Oyster Card.
www.tfl.gov.uk