1 Correct answer
Math speaks for itself.
Using variable instead of length is more than 20 times faster. So, conclusion, undoubtedly, is that setting variable to array length and then using it in a loop is the way to go, especially on large iterations.
Here is a benchmark test:
Without length:
var st:Number = getTimer();
for (var i:int = 0; i < 1000000; i++ ) {
}
trace(getTimer() - st);
Trace returns something around 12 ms
With array length:
...var array:Array = [];
for (var j:int = 0; j < 1000000; j++)
{
array.push(1)
22
Replies
If you use Array.length as for loop condition Flash has to evaluate Array.length on each iteration again and again. This is slow, waste of resource, and not elegant 
There are no difference in the code "readability" here so there are no reasons you should use inefficient method specifically.
Using Andrei's example (but with a larger number):
var array:Array = [];
for (var i:uint = 0; i < 9000000; i++) {
array.push(1);
}
var st:uint;
st = getTimer();
for (var j:uint = 0; j < array.length; j++) {
// loop 1
}
trace("loop 1", getTimer() - st);
st = getTimer();
var len:uint = array.length
for (var k:uint = 0; k < len; k++) {
// loop 2
}
trace("loop 2", getTimer() - st);
// trace
// loop 1 682
// loop 2 112
If you store the length in a var it's much faster.
How I usually do is to create a var to store the length in the for loop initialisation so the code is cleaner:
st = getTimer();
for (var l:uint = 0, len2:uint = array.length; l < len2; l++) {
// loop 3
}
trace("loop 3", getTimer() - st);
// trace
// loop 3 111
The speed is the same as the loop above.
You can also assign the length to the iterater at the initialisation, then subtract 1 on each iteration:
st = getTimer();
for(var m:uint = array.length - 1; m > 0; --m){
// loop 4
}
trace("loop 4", getTimer() - st);// trace
// loop 4 118
I'm going to have to disagree with Andrei1 - You're right ryancanulla that in your case, it's an insignificant difference in performance, and his numbers are very bloated by an unrealistic situation (ahh, statistics, how malleable).
Also, readability is HUGE - when it comes to speed, and *especially* memory, for the kinds of applications that will be created in AS3 it is WELL worth making small (imperceptible) performance sacrifices to make code more readable. ANY software engineer worth his salt can give you an earful about the importance of organized and readable code. While yes, in some systems you want to eke all the performance you can, creating a flash app is NOT one such system.
Just backing you up ryancanulla, you made the right call 
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