SRT has almost no styling or positioning, and character encoding problems are common. SMI can carry several languages in one file for the players that read it. SRT is at home in subtitles that must work almost everywhere; SMI is the usual choice for subtitles for Korean-market players and multilingual files.
The text is read as UTF-8 or UTF-16, whichever the file is. An older file in another code page is read using the charset it declares, or as Windows-1252 if it declares none, so accented letters in an unlabelled legacy file can come out wrong. A file that is not text, such as the picture-based subtitles of a DVD, is refused instead of being converted to nonsense. Of the styling, only italic, bold, underline and the position on screen (top, middle or bottom, left, centre or right) travel between formats; fonts, colours, outlines and effects do not. The file is written with one language class, and cues that overlap in time are shown together, as SAMI has a single timeline.