what is the difference with sequental and non sequential turbos? which ones are better and why?

 

The obvious answer is that when sequential, the turbos are operated sequentially. When parallel, they are operated in parallel.

The stock FD use the primary turbo at low load to provide instant boost. As airflow and load increases, the secondary turbo is brought online as well. This is sequential.

Going parallel just means converting it so both turbos are used together.

There is no "better". Each one has advantages and disadvantages.

When it works, which is rare, the stock sequential turbos are wonderful. But because the system is complicated and unreliable, many people choose the simple parallel setup as an alternative to having their turbos broken every 5 minutes.