I don’t think there is a clear answer as to which one is “faster.” It used to be that CP is slightly faster than AOS, but for a given case, it can happen either way. It is especially true nowadays that backlog (retrogression) and security checks account for the big chunk of waiting time, so once you become current, the difference between CP and AOS is probably negligible.
If I were you I’d put more thoughts into other factors. The majority of applicants, if eligible for both, would go with AOS for the apparent benefits such as EAD, AP, no travel abroad, etc.
