I can see why you like the cosmos II, similar to XL, has 6 hdd bays on the bottom and i think 5 on top. But the plus that i do see on the cosmos II is that if the hot plug slots are removable and you end up with pure 2x 5.25 slots you could install the fan controller + optical drive + 4n3, this is if the 2x slots are removable. I would probably wait a little because of Lawrence Lee comment,
My guess is SPRC recieved a sample, as so many sites just got one, so we might see a review from SPRC soon, then you can see if you go with the cosmos II or the XL.
I am very picky, because I am still stubborn enough to want only 1 machine, and use it for gaming, overclocking, daily use, server, and keep it silent.
Personally i would recommend against, i would prefer to have a separate server accessible via network, that way you can even place it outside your hearing range, 14 hdds will be noisy regardless of how the setup ends, specially if you use a raid array and uses a lot of drives together. A seperate server also allows you to use an OS more oritented for for serving, backup, etc, like for example unRaid that allow to spin down drives that are not in use, you can also use a very low CPU like an atom (well depends on your needs), but this way you also upgrade when you need it, and leave you normal PC with a SSD and if you want a local single hdd for local storage, this would end up imo as more quiet setup. But here is more about personal needs and preferences.
Good luck with your choice.