I would have thought you would get your best cross-platform compatibility with the TrueType. The message you are getting about the OpenType-TT is because it's format is not understood by FM to be an english font.
Windows pinches pixels... always has! There is always some amount of shift and in windows the text object usually has to be a bit bigger to display the same text. (Disclaimer: I havent' tested with v12 of FM and Windows 8)