articulate the differences in these two snippets. which one is "easier"? which one is "simpler"?

gistfile1.js

function XYZ() {
   X();
}

function X() {
   // do X
   Y();
}

function Y() {
   // do Y
   Z();
}

function Z() {
   // do Z
}

gistfile2.js

function XYZ() {
   X();
   Y();
   Z();
}

function X() {
   // do X
}

function Y() {
   // do Y
}

function Z() {
   // do Z
}

I would prefer the second one, it gives the right picture when I look at XYZ and then summarize on what it does and how it does things..
Easy to understand and easy to explain ...

The dependency graph is totally different in both.

In the first one X is dependent on Y, Y on Z etc.
In the second one X, Y and Z are totally independent of each other.

I'll prefer the one which actually reflects the business domain correctly.

#1 makes me want to die.

(xyz (x (y (z)))
(xyz (x) (y) (z))

Both are wrong. In a true enterprise application, function XYZ would be written as

function XYZ() {
  // do X
  // do Y
  // do Z
}

(Note: tongue planted firmly in cheek)

The latter!