I am in process of sponsoring my parents and had a quick question.

Should i apply for their visitor visa first and then apply for immigration.
The reason i am asking is because i heard that if i have already filed for their immigrtion then getting a visitor visa is not possible.

And i want them to visit us this summer, Pleasea advise.

Depends but possibility is there of a visitor visa refusal. So you are better off applying after they get visitor visa.