What you will get is opinions and everybody has one.  Both are good guns.  MAgs for M14 are pricy, for good ones at least.  FAL mags are dirt cheap and very reliable.  FAL parts are, for the most part, cheaper.  FAL is easier to clean and service but that is avery personal observation.  I have had both.

Either will shoot darn good with decent ammo.  Either will mount the scope of your choosing.  FAL is much easier to adjust headspace if required and is not plagued with the problems of maintaining headspace that some of the Chinese and Frankford Arsenal guns are.  Truely good receivers for M14 are Springfield and ? as no originals can be had (full auto) nor have any EXACT licensed copies been made that I know of.  Actually the Polytech is an EXCELLENT receiver but the bolt sucks!  You can easily wind up with $1100 up in a good M14.  A FAL will set you back $450 up.  DSA are outstanding but they ain't cheap!!  A parts gun will shoot nearly as good as the DSA and is a good bit cheaper, say $525 up.  DSA LOOKS great, shoots great; depends on budget and just what you want.  Parts guns are as good as the parts used.  If you buy a cheap kit with poor barrel then you know what to expect!  Before paying big $$$$ consider a preban.  Not the cheapest rout but will hold value well and is ALL mil-spec goodies.