Spousal visa VS permanent residency

To the best of my knowledge, there is no 'Spouse Visa'.  The nearest equivalent is the 'Dependent Pass' which you already know about.

Permanent residency (PR) is what it says ............. the right to remain in the country on a permanent basis and includes most of the rights of citizens too; so you can work, for  example.  You must continue to live in Kenya, as you main country of residence, so PR isnt right for people who want to divide their time between two countries.  I am sure that I read that PR can be withdrawn if the person starts to spend more time out of Kenya than they do living there.

To apply for PR, you must have been married (to a Kenyan citizen) for minimum 3 years and the marriage must be enduring and ongoing.  A current Certificate of Good Conduct from Kenya Police is needed and you can only obtain this by holding one of the temporary permits (plus Dependents Pass, if eligible).  So, as you can see simply being married to a Kenyan isnt going to automatically eligible for PR, without going through other steps.

I understand that the application for PR can take a year or so and to remain in Kenya during the period of application, you must have a valid visa or pass to remain in Kenya - simply being married doesn't give the right to remain if your other visas or passes have expired while you were waiting for the PR application.