The question between triangle/quad/polygon is ruled by the polygon stop flag in the M15 file format. Of course, you could dived it into triangles all the time, if you know the right algo.
In which format are you storing the datas for a model inside your delphi app? That should give you the answer anyway. If you plan on importing over file formats, then you might need several ways to store them or generally store them as triangles.
The strip versions are mostly used for speed reasons. But I don't have enough knowledge about this to tell you when to use strips and when not.
If you use polygons, then you are save, as they can easily have 3, 4, 5 sides.
My guess for TBGL is that Petr uses Polygons as a M15 model can have triangles and quads.