Let's see....
Best: M14/M1A
Best value: FAL
Most 'gadgety': Armalite AR-10
Least expensive, without getting 'cheap': CETME
Personally, I am waiting until Fall, 2004, to see what happens with the 1994 Assault Weapons Ban, since it expires at that time. If it looks like it will not be replaced, there is no reason to pay more for a 'pre-ban' rifle. So I will be investing most of my 'gun money' in my M17S, and putting some away each year until then, to get an M1A with all the trimmings.
To answer a few questions:
"Functionally, what differentiates them?
What makes one a M1A and the other a FAL?"
The methods of operation are quite different - I don't think the details are really that important, here. Suffice it to say that the M14/M1A has an edge-up in reliability and accuracy, but is also quite a bit more expensive, both in initial price, and in purchase of magazines and such.
"What are the functional issues, pistol grip versus rifle stock?"
I think this is like the 'revolver vs. auto' debate - everyone has a preferrence, and is usually convinced that his is the best way. Generally, it seems to me that competition shooters prefer pistol grips, or semi-pistol grip (imagine a pistol-grip shotgun, with a buttstock attached low on the back of the grip, or pick up a copy of the August issue of American Rifleman, which has one on the cover, as well as a series of articles on the M14 and M1A)
"Can you get a M1A with a pistol grip?"
Stock, no. But aftermarket stocks in a variety of forms are available. Additionally, a variant of the M14 was produced, called the M14A1. Its stock had a pistol grip, as well as a folding forward grip for the off hand. These stocks are occasionally available. Of course, they are expensive, as with all other things related to the M14.
So, FAL is probably your best bet, for a good value. The M1A is much more expensive, but is arguably the most accurate 7.62mm service rifle ever fielded.
Speaking of caliber.... remember that .308Win and 7.62NATO are not the same - a .308Win chamber can safely fire all 7.62NATO ammunition (assuming the firearm, itself, can take the mil-spec pressure), but a 7.62NATO chamber may not be able to safely fire all .308Win ammunition. From what I recall, if you wish to use .308Win ammunition in a 7.62NATO rifle, you need to get a .308Win headspace gauge, and make sure the headspace is lower than 1.636-1.638
--JB