I've loved the Invisible Deck since I learned it years ago. I've recently begun to really appreciae Jay Sankey's X-Deck. Would be interested to hear what your preference is. Invisible Deck or X-Deck and what have your spectators reactions been.

What's the effect or difference between the two?

Same deck. "X-Deck" is invisible deck with 'X' marks on the faces of one side of the deck (i.e. the odd side). If a spec chooses an odd card, it's not only the only one face down but it also is the 'only' card with an 'X' through it. Personally I'm not a fan of the 'X' deck because the other half of the time you're showing that every face has an 'X' through it EXCEPT the chosen card. Not nearly as good of an effect.

I prefer the standard invisible deck because it gives you a little more flexibility as far as routining or special uses like using it as an out, etc.

I like the X deck, has a nice appeal to it, and is very simple, therefore very effective. But the invisible deck works wonders. What fan do you guys use? I like to use a one handed fan.

Somewhat different effects. I have actually had people suggest sleight of hand with the invisible deck. I somehow found and turned their card over. The X deck cancels out the sleight of hand idea. Of course
the invisible deck should also if anyone was actually thinking.

I like the x idea. Now I use half of them red and half of them blue... more distinctive to me.

The two colored Xs that AaronishMagic suggested would give you a logical reason to set that deck aside and carry on with another deck.