@cybersurfer - I would say it really depends on your case. You are on L1 now, if you go for Visa stamping for H4, then you will not be able to continue working in L1. So if you want to start H4 processing now, then COS would work for you. In my case, my employer pushed me to apply for H4 COS so that they can apply for EAD and asked me to continue working till L1 expires. (which expired last months and I am in painful waiting now)
Regarding timelines, it is pure luck. When my spouse applied for H1 Extn in Dec 2016, Vermont had huge delays in H1 processing. So he filed H1X in Nebraska. But in three months it turned upside down, people applied in Vermont after him got approval first. My point is that nobody can predict what would be rate of processing after two-three months.