Permanent pet spider: Pros and cons
Solution 1:
Permanent is relative here, if you ever feed your pet another transformation fish he'll go back to his original form once that one runs out.
The only advantages I know of it is that you can then sell these webfish (for whatever they earn you) and you don't have to micromanage your pet eating things anymore.
The default pet has a small disadvantage to the "fished" up pet in all cases, the spider pet has his web, the crab his bonus defenses,...