For those of you who have seen or own both the gloss and matte finish ACKS hardcovers, which do you prefer?
Gloss.
Whichever matches the existing books. Barring that, whichever is more durable over the long term.
seconding what bob says
Gloss.
I’d vote for durability as well - I’m guessing my books are gloss \ 1st printing from the kickstarters, but I’d prefer matte. I do have an oddball question about the core ACKS book, though - have you ever thought of a ‘deluxe’ edition that includes a page marker ribbon? Just asking.
Definately matte, it won’t show all the travel scratches. My player’s companion is starting to look beat up, but my second-printing core hides it’s scratches very well.
I have thought about many such things but my little company’s production capabilities are 1st level while my imagination is 14th level! 
Gloss
I have the glossy version. I think I’d prefer the matte.
Very much matte. I have gloss, but a couple of other folks at my table have the matte printing and it looks much nicer to me.
(That being said don’t read too much into my “very much” I like the look of the matte books better but it’s not THAT important!).
Very interesting to hear people’s positive experience withe the matte cover. I would figure matte attracts dirty paw-prints easier. I have my 1st edition with official Tavis beer stains and I superstitiously believe the gloss cover helps it avoid scratches while traveling in the sling-pack.
I have the matte Core book and the gloss Player’s Companion. I like the look of the matte finish more.
As to durability, I haven’t had the hard-copies of either one long enough to verify.