SBV is understood mainly by YouTube and a few caption tools. ASS gives full control over how subtitles look. SBV is at home in uploading and downloading YouTube captions; ASS is the usual choice for anime and karaoke subtitles with fonts, colours and positioning.
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 a single Default style (Arial, white with a black outline) that the player or Aegisub can restyle, and times are rounded to hundredths of a second, the precision ASS has.