Quizá esto no se aplique a España, pero en mi opinión si el texto se refiere a una marca estadounidense que lleva TM, yo dejaría TM. TM y R no son lo mismo, por lo menos en EE.UU. TM es una marca comercial no registrada, mientras que R es una marca registrada.

Aquí va una buena explicación que encontré:
Although there is no difference in the rights granted to owners of trademarks versus service marks, the type-distinction is made in the application for registration and is carried through all records of the mark to the registration, itself. In providing public notice of use as a trademark or service mark, unregistered marks should carry the ™ symbol when they are used as trademarks and the SM symbol when used as service marks. All U.S. registered marks carry the ® symbol

--------------------------------------------------
Note added at 2002-03-04 22:58:05 (GMT)
--------------------------------------------------

Otra explicación que encontré en http://www.chinahk-ip.com/TMFAQ_hk.htm: TM stands for \"trademark\". SM stands for \"service mark\". Anyone who claims rights in a registered or unregistered mark may use the TM or SM symbol as applicable. However, such claim may or may not be valid. The R symbol may only be used when the mark has been registered. Falsely representing a mark as registered subjects the offender to a fine.