Composite Primary keys and auto increment?

i'm developing SaaS app with multi-tenancy, and i've decide to use single DB (MySQL Innodb for now) for client's data. I chose to use composite primary keys like PK(client_id, id). I have 2 ways here,

1: increment "id" by myself (by trigger or from code)

or

2: Make "id" as auto-increment by mysql.

In first case i will have unique id's for each client, so each client will have id 1, 2, 3 etc.. In second case id's will grow for all clients. What is the best practice here? My priorities are: performace, security & scaling. Thanks! Same question at stackoverflow with more answers