It is really a personal preferance, but I will give you a little info to go on.
Exile is amazing and the reactions it gets are better (imo) than the reactions for x-finger, however it requires a lot of setup and is not very practical for what I do. It has some flexability, but you will want to stick with the method that is taught because it is the best.
X-finger is fun to do, and completely impromptu. If you can find a pen, you can do x-finger. I like x-finger better because it uses no gimicks and audiences like it. It has tons of versitility, you can come up with many effects of your own using this concept and they give you several on the DVD. You can use it to transform words or reveal predictions from nothing.
So, I prefer x-finger as a stand alone because it is so adaptable and requires no setup, but one thing to think about is these compliment each other very well. If you take the gimick(s) from exile and use them in an x-finger routine it can be a lot of fun. I use both things together for a routine where I draw two verticle lines on my finger, then a little wave and they cross to form an X. Then the X dissapears. I pull borrow a coin put the coin in the specator's hand and draw the x's for exile, then 'pluck' the x's off and they're under the coin. Very good reactions from that one.
Good Luck,
Josh