Training 6-star is slightly slower than 5-star, it changes significantly at 7th star... imo, it depends of the player position... if it's MC, no SA will help him play better, so another position might be useful... if it's defender, I prefer Aerial Ability SA over Defensive Wall SA... and if it's attacker, Free Kick SA might be useful addition to one of them who is consistent performer... for wingers I prefer Corner SA, 'cause they are usually short and not useful in the box... anyway, if a player is poor performer, nothing will help him to play better... Some of SA are useful for some type of players, but most of them is waste of skill points...