I have the option to install either a MOE or aluminum trigger guard in a new build, and I'm wondering if there is a specific advantage to one over the other.

My initial thought was that the aluminum would be more robust, but polymer is pretty hard. Plus, if the trigger guard is hit with enough force to cause damage, I figure most of that stress will be concentrated on the attachment points. Isn't the retainer screw or pin more likely to snap in that case? If so, then it wouldn't matter which version (polymer or aluminum) I was using, right? Is my thinking wrong on this?

I kind of want to go MOE and save the aluminum. Of course, the MOE is NIB and the aluminum was previously installed and then removed.

Personal preference.  Choose what you like the most. Or toss a coin.  I use a magpull guard but if i had a nice machined metal part i would most likely use that.

They're both Magpul. I guess I may as well go with aluminum for the 'wow' factor.

IMO, if the guard receives a blow significant enough to break the polymer guard, it could be significant enough to bend the aluminum one... What is less convenient for you?  A guard that's broke, or a guard that's bent?  

There was a time I had all Bushmasters and I had their cheesy Mil-spec looking plastic trigger guard break when my rifle was riding in a rack on an ATV.  I then swapped all of the plastic trigger guards with mil-spec aluminum.  My dad and I went rounds with his opinion that the aluminum could bend and impede the trigger's function.  I said "F" that, it would be rare and if it happened, I'd swing the guard down and go on firing if required.  The MP plastic isn't thin like the BFI abortions.  I'd take either one.  If it were my keeper rifle, I'd definitely install the aluminum one just because.  I won't say it's better, but it's pretty damn robust.  If I may send it down the road, I'd put the plastic one in. YMMV & HTH.

Your over-thinking it.  A breaking trigger guard won't stop the rifle from functioning anyway.  Just get whatever you think looks best.  I like the polymer ones because they cost a lot less and can be color matched to other furniture.

The Bushmaster trigger guard is a carbon fiber, not standard plastic.  Just fyi.  I wonder how magpul polymer holds up under extreme cold?

Aluminum Magpul Trigger Guard here for $12.33 shipped.