Sights, trigger, and (arguably) safety are better on the M1A, plus it's a little more...."patriotic", if you will, being such an American icon. An M1A will eat any surplus 7.62x51mm or commercial FMJ .308 ammo you throw in it with no complaints, while a FAL will make you adjust the gas system for different loads. It also strikes a near-perfect balance between the "totally evil" and "toatlly normal" rifle "looks": everyone won't look at you funny if you take it hunting, but it's just "evil" enough to fit in equally well at any Arfcom-ish shoot. Considering practical matters, an M1A (if it's not a Norinco copy) will never give you trouble in the parts count department... i.e., you never have to worry about how many US-made parts are on the rifle, since they're all US made to begin with. On a FAL, however, the opposite is true. For example, it is relatively common for FALs to have the mag floorplate be one of the US parts. That means every extra mag you buy will have to have a new, US made floorplate installed. This may or may not be a hassle, depending on what parts a particular FAL has made in USA and what you plan to do to it. Also - in case anybody besides me cares at all - the bayonet for the FAL looks stupid, especially when attatched. The bayonet for the M1A is uber-awesome - especially when attatched (I have a "thing" for bayonets...).
Don't get me wrong, the FAL has it's own pluses, too. I just have to manifest my M1A bias at all possible opportunities...
FAL overall length: 43"
FAL barrel length: 21"
FAL sight radius: 22"
Weight: 8.75 lbs
M1A overall length: 44 1/3"
M1A barrel length: 22"
M1A sight radius: 26 1/2"
Weight: 9.2 lbs
As you can see, there really aren't many differences in terms of dimensions: only a 1" differenece in barrel length (M1A's favor), 1 1/3" OA length (FAL's favor), and a little less than a half of a pound difference in weight (FAL's favor). The only important difference - sight radius (a rather large 4 1/2" difference) - is hands down in favor of the M1A. This, combined with the fact that the sights are better to begin with and the presence of an already slightly longer barrel, gives the M1A a slight edge in practicality IMHO. The M1A really is a better target shooting rifle and, at only 1" longer than the FAL, can still do all the "nitty-gritty" battle rifle-type stuff that the FAL does.
IMHO, the only real benefits the FAL have are a pistol grip and cheap mags. Most every other comparison either comes out a tie or in favor of the M1A, arguably.