sword speed

Some swords require higher swordsmanship but do less dmg then other swords, is there some advantage in attacking speed that makes them better? For instance: what is the advantage of a 14-20 spatha (req. 10) over a 15-21 fellblade (req. 9)?

There is no advantage. All swords attack at the same speed, bows however are a different story. Each type having a different range, speed, and rate of fire.

So there's no difference at all between spatha's, fechions, tribal blades, fellblades, winged swords, long swords and whatever you may have..?

Nope, the only difference is purely cosmetic.

that's why I wonder why some weapons have the same skin but different names
and some have the same name but different skins