Do suspension seatposts work as well as a full suspension MTB? I used to have department store (Target) MTB that i used for 2 years until it got stolen, i used to ride casually in the city no complaints as of reliability.

Want to buy same because we have very bad roads here where i live and i am tired of going over bumps on the city bicycle.
Where i live its tough to find used MTB with built in rear shock, would i get equivalent suspension if i buy hard tail MTB and upgrade to seat post with shock compared with bicycle on picture?
Suspension seat posts and full suspension have different jobs and can't be compared.
The primary goal of a suspension seat post is to provide rider comfort.
The primary goal of bicycle suspension is to keep the wheels in contact with the ground to increase grip and control over rough terrain.  This has a secondary outcome of increasing comfort.