I did a search on youtube and found a small handful of videos .. 2 used plaTINum, one was silent, one used both, and one used plaTIGnum but said he had no idea how to pronounce it.
I would probably go with SBRE Brown because he has probably dealt with the actual company and he used platTINum.
Personally I will probably stick with plaTIGnum to differentiate it from Platinum pens.