L1 process is faster, person can come to US sooner.

After arrive in US, then start to apply for GC.
With L1, no LC/Perm is needed, can submit I140 right away.
But in order to apply for L1, the applicant need have worked for some company at management position or owned a business in oversea for one or two years, and need setup a sub-company in US or transfer to existing sub-company in US for applying L1.