It seems that many people still don't know the difference between ® and TM. Its quite self explanatory when you know.
® signifies a registered trade mark, its one that the owner sees as valuable and has spent money to protect it properly. It is likely that the owner of this mark will exert their rights rigorously.
TM signifies an unregistered trade mark. It simply means that the owner is seeing this as his brand. He will have unregistered rights under the law of passing off whether he uses this symbol or not and as such I often advise people not to use it as it is highlighting that you don't value your brand enough to protect it.
Should you register your trade mark? Good question, if you would be worried about a third party using a similar mark then yes without question. Passing off is expensive and difficult to prove, having registered rights makes taking action against a third party much more straightforward.