Hey there!
I'm a long term customer of Ellusionist, and this is my opinion:
Best for Magic: Black Tiger
Black Tigers are thick and glide smoothly. They last a long time and help you to accomplish sleights with ease. Also, double lifts and grouped cards are harder to spot because the black just blends in to the other cards in the deck.
Best for XCM: Vipers
As almost anyone can tell you, Tally Ho's have always been a favorite in the world of XCM. They have a substantially better feel than bikes while flourishing, and also have better snap, finish, and durability than normal bikes. But when you get into the Viper territory, your Tally Ho's just achieved a smoother finish, slightly thicker stock, and improved lifespan. In addition, they fan beautifully.
Best Balance for BOTH: Ghosts (or Black Ghosts)
Ghosts have the smoothness and thickness of the Black Tigers, but are still good for cuts and packet flourishes. Because they lack that black ink all over the cards, they are also somewhat thinner than the Black Tigers, which makes your cards more lightweight and easier to flourish with. They group together better than the Black Tigers, as the Black Tigers seem to fall apart while floushing while the Ghost do not. Finally, they seem last the longest out of the other E decks. Black Ghost are the EXACT SAME as the regular ghosts, so your decision between the two should be based purely on preference of color scheme. (However, keep in mind regular Ghosts are much cheaper).
If you are still not sure, try this sample pack out. It has all three of the decks I recommended:
http://www.ellusionist.com/product/playing+cards/bundles/custom+3-deck+sample+pack.do
Hope I helped!
EDIT: Yes, they still have their 6-deck sample pack, which can be found here:
http://www.ellusionist.com/product/playing+cards/bundles/custom+6+deck+sample+pack.do Keep in mind this sample pack includes all of the custom E decks, EXCEPT the Black Ghost deck.