R is a federally registered Trademark, that has gone through review by the United States Patent and Trademark office, these jokers
TM is a common law trademark, a self issued mark where your rights as a trademark holder have quite a bit of backing by federal law. By default anything company X markets, they have trademark rights over what they call it no matter whether or not they bother to get a federal mark. Stamping TM beside it is a sort of public disclosure that indeed you are using that name as a mark.
Yes, of course a big fat Federal 'R' is stronger, but that takes a while to get (6-10 months ballpark) and runs about $700+ give or take by date of issuance. This buys you 1 decade though...
Of course, I'm not a lawyer, I could be getting a few details wrong, check out the USPTO site for a definative answer, but not a fun web browsing experience.