Before posting, please... PLEASE don't just type as fast as you can and post, re-read what you posted because it irks me ever so.
(Devine/Divine, im/I'm, gona/going, etc)
However, to answer your question, there's multiple reasons.
1. The regular attacks are exactly identical between Sealed Sword, Divine Avenger, and Gran Faust. The only difference is damage, and damage type.
2. Faust and Gran Faust are the only weapons that inflict curse in the game. Period. They inflict it with every attack, regular or charge, at a decent rate.
3. There's also the matter of the fact that it inflicts Shadow Damage, which is the least common damage type you can inflict. The only ways of doing so are Faust, Nightblade, Blackhawk, Shadowtech Alchemer, Toxic Catalyzer, Graviton Bomb, and their upgraded forms.
Of those, Blackhawk deals split pierce/shadow making it less desirable, for one.
Secondly, Nightblade and Faust are the only two swords that inflict shadow damage.
There's more, but if you even merely took from the above that it's one of the two swords in the entire game that inflict shadow damage, as well as being the only weapon to inflict curse... It should be obvious.
If you think GF is slow, then you will be pissed at the avenger line.
Faust line are good for JK run. Since the normal 2nd swing doesn't curse you but does curse on your target. So if you planning on doing more JK run, get the faust first so you can solo JK easily with it and get more token to get yourself an avenger as well.