I would go with the Burris. I have a Leupold 2X7 on my L1A1 and do just fine. You just don't have to have mil dots.
Short of an ACOG, the best rangefinding optic with a BDC, as well as the toughest and most reliable optic in existence, are your factory iron sights! The FAL has really good aperture sights that are adjustable for range, which are the envy of lesser battle rifles that only have notch sights and/or a really short sight radius.
One of the things you will learn at an Appleseed is how to use your front sight post to estimate distance to target based on the width and/or height of the post, and to figure out your come-up or hold-off.
So being that you already have one of the best and most effective combat optical sights known to man attached to your rifle, what I would do is practice with your iron sights while you're saving up for that ACOG... and even after you get your ACOG, pull it off every once in a while to keep up your skills with your irons. You know what they say -- "2 months after the SHTF, iron sights will rule the world," or something to that effect!
All in all I get the feeling that A)the burris would be a good buy for now but B) an ACOG is worth saving for and purchasing later, but I should keep in mind C) that training with my irons is still going to be a neccesity (goes without saying).
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