I have personal experience with 2 of the 3 in the OP, as well as an FAL. My personal preference would be
1. DPMS LR-308 - Awesome ergonomics, super accurate, compact, lighter than a PTR or FAL (dont know about Saiga). Easy to mount optics, very modular. Only downside is the high cost of mags. I did as JWARREN did and paid and waited for an order from C Products. Took a couple of months but at $22 ea they are a great deal. I really should get a bunch more. The AR-10's also have a reputation for having reliability issues, but I think that is in large part due to the older Armalite problems, and the Bushmasters. My DPMS has been perfect so far, but I am using it more as a DMR rifle so it has not been fired hard like a "fighting rifle", and I have used decent ammo and mags in it.
1A. PTR-91 - To me it balances much better than the FAL. SUPER reliable, mine even eats the Indian surplus ammo without any problems. Magazines are dirt cheap, tough, and easily found. The PTR is a heavy sucker, but for a fighting 308 it would be my number one choice.
3. FAL - I had one and sold it. It did alright, but I did not like the way it felt, the ergonomics and balance were very wrong for me. If I found a great deal on a DSA FAL with an 18" barrel I would be tempted to give it another whirl.
4. M1A - Never owned one and dont have much personal experience. The one I shot did have great sight radious and a decent trigger. High cost of mags, very high cost of the rifle, lack of a pistol grip, and questionable reliability (in my limited experience), push it this far down the list for me. I know you can spend a lot of $$ tricking them out with different stocks and grips, but I think that spending that same $$ on my DPMS gives me more bang for my buck.
5. Saiga - These rifles sure are popular with a lot of people, but they dont do a thing for me. Mags are high and hard to find, have to spend time and money changing to get a pg, etc...I have a buddy that is a huge AK fan and even he doesnt like the Saiga. A lot of people do though, and I suppose they are very reliable. I think a lot of people love them because of their low initial purchase price. To me though, the extra that you pay for a PTR over a Saiga is quickly lost in the cost of mags and ammo that accompany the rifles.
All the above is JMO. Almost none of it is scientific fact