others more literate in computers will step in i'm sure, but i can offer partial answer - m.2 SSDs are PCIe interface which can go up to 32 GB/s transfer speed where 2.5" SSD is sata interface which is limited in sata III to 6 GB/s
if your M.2 socket on your mobo is listed as 10GB/s then it's only using 2 lanes of a potential 4 lanes. Some M.2 SSDs are only 2 lane capable (iirc). A couple of the Z97 asus boards actually have 4 lane sockets, but the z99 boards generally have the full 4 lane sockets.
My asus board (Z97M-Plus) has the 2 lane M.2 socket - the other disadvantage to using that socket is it kills two sata ports (5&6) on my board - i suspect it will do the same on yours - read your mobo owner's manual, if it sez "M.2 shares bandwidth with sata 5 & 6 ports", technically asus was accurate in that descript, but that sounded better for marketing purposes than saying "you can't use both at the same time, and using the M.2 socket will automatically dis-able sata 5 & 6).
You can mount it in a PCIe expansion card and get the full 4 lanes if the PCIe port is "x4", like kingston does on the kit they sell with the card included (http://www.thessdreview.com/our-reviews/kingston-hyperx-predator.m-2-pcie-ssd-review-480gb/.
Or you can buy an expansion card separately - here's one
http://www.addonics.com/products/adm2px4.php
but in short, the M.2 is decidedly faster, even if only used in the M.2 socket - when i installed it in my M.2 socket, boot times dropped to 10-12 seconds from 20-22 seconds. I've ordered the expansion card and hoping to see boot times drop further
hope that helps
PS - for a smoking fast setup, think two M.2 SSDs in Raid 0 http://www.thessdreview.com/our-reviews/kingston-hyperx-predator-m-2-pcie-ssds-in-raid-0-you-thought-one-was-fast/
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