I have 2 .308 saigas. One with an aftermarket folding stock. I have 3 FALs, A STG 58, L1A1 and C1A1. All my FALs were built on metric recievers. two by Century and one by a private smith. All have been extremely reliable.
The downside to the Saigas:
1) The accuracy is mediocre (but better than any other AK I own)
2) Magazines are expensive ($40 ea).
3) The short barrel doesn't allow it to make the full muzzle velocity that the .308 is capable of.
4) AK ergonomics are mediocre at best when compared to any rifle except the CETME.
The Saigas upsides are:
1) The folding stock was easy to install
2) The scope rail is effective and easy to mount a scope to
3) It is a NEW gun, still in production and every saiga was built at the factory.
4) there are alot of accessories are available specifically for the Saiga and for AKs in general, most of which will work on a Saiga.
FAL Downsides:
1) Not an easy rifle to mount a scope to
3) Not easy to install a folding stock.
4) Long barrel is unwieldy in a building or vehicle.
5) Every FAL I've seen is a mix of new and surplus parts. A all new, never issued rifle would be very expensive.
FAL upsides:
1) good ergonomics
2) lots of available parts and accesories
3) good muzzle velocity
4) relatively cheap magazines
Of the two rifles, here's the one I would rate as superior in each category and to what extent:
cost: Saiga +++
Reliability: Saiga (barely)
ergonomics: FAL +
accuracy: FAL (open sites), Saiga (Scope)
All that being said, if I were to go to war with one of my .308 battle rifles, the order I would choose them would be:
.308 AR 15
FAL
M14
Saiga
CETME