Yes Sata III = 6.
As to which is fastest. You would have to look for a specific Test showing the two.
On boot time I'm quesing the m4 would be faster as it has Higher 4 k random reads I think. How often to you boot, saving two sec on boot is not much as probably not do much in two sec. I normally turn min computer on and walk a way and come back - And it's not because of boot time.
If working with small files, then Random is very important. when working with Very large file stuctures - Some photos can be 10 megs, a video .VOB is typically 1 Gig, and a blu-ray video can be up to 40 gigs (Watching it is no problem, you can do that at the really slow speed of a DVD, its encoding) these need High Sequencial performance. Reads are more common than writes.
Bottom line - Unless some one has both, it's going to be hard to get an honest answer.