The format of outlines is only difference between .ttf and .otf, yes. .ttf has quardratic Bézier curves and hinting is done by interpreted programs (on Windows;

The format of outlines is only difference between .ttf and .otf, yes. .ttf has quardratic Bézier curves and hinting is done by interpreted programs (on Windows; TrueType has generate hints at run-time). .otf has cubic Bézier curves and declarative hints. The outline format of .otf comes from Adobe Type 1 fonts (.otf really is just .ttf with outline format taken from Type 1), so the .otf outline format is the older one. Apple created TrueType after licensing talks with Adobe failed long ago. .otf is generally more compact in terms of font file size than .ttf. Really the only reason to stick with .ttf is the rasterizer situation on Windows and outlines traveling from Linux and Mac to Windows in PDFs.