In a way, yes, the Flayer is better than the Venus Gospel as it has materia growth. Even thought the Flayer has more attack power, the Venus Gospel is more powerful. This is because the Venus Gospel powers up when Cid has a lot of MP. The Flayer is a very good weapon to use before you get the Venus Gospel, but it is not more powerful.

Venus Gospel: Given to you by the Old Man outside the Materia Shop in Rocket Town, after the rocket has been launched.
Attack: 97
Attack %: 103
Materia Slots: O=O O=O O=O O=O
Growth: None
Notes: This weapon powers up when Cid has more MP. The power decreases as his MP does.

Flayer: Exchanged for 5,000 points at the Shooting Coaster in the Gold Saucer (Disc 2 and 3)
Attack: 100
Attack %: 100
Materia Slots: O O O O O O
Growth: Normal