Looking to expand my core set and can't make up my mind. Should I get Order of the Sliver Twilight or Secrets of Arkham?

You should either get the (reprinted) Secrets of Arkham or the Terror in Venice deluxe expansion. Either one will bolster all of the original factions which imho is more important than introducing an eighth faction.

Thank you for the input. I mainly play with my wife and we want to get our core set a little more variety. So if you had to choose between Secrets or Venice, which would you do? I was thinking with going with Secrets because it was originally released before Venice. What do you think?

Get both!

It's a tough decision. With Secrets you get a new set of stories, with Venice you get a bunch of interesting conspiracies and Day/Night cards. Getting Venice would probably have a bigger impact on your games, but also includes some cards from the Order of the Silver Twilight; adding the cards from Secrets would be a more gradual change, slightly bolstering existing themes.

Hmmm... I did purchase the Shifting Sands with my core set (I needed the free shipping! ) So I am good on new stories, but I think SoA is the best route to beef up my core set. Is there any Asylum Pack you would recommend to order with SoA?

Well, since you already have one AP from the Ancient Relics cycle, you may want to get another one from it.

My suggestion would be 'Into Tartarus' which contains several powerful 'staple' cards:

http://www.cardgamedb.com/index.php/CoC/CoCCards.html/_/ancient-relics/into-tartarus/

Alternatively you may want get one of the APs from 'Forgotten Lore', since that's the only cycle in which each AP has a self-contained theme. E.g. 'At the Mountains of Madness' is based on the story of the same name and mechanically introduces the 'Polar' subtype:

http://www.cardgamedb.com/index.php/CoC/CoCCards.html/_/forgotten-lore/at-the-mountains-of-madness/

Thanks again for the input.