Having owned both (Saiga 308, just got rid of it) and now a SAI Standard, I will have to say that I would trust my life and my families to either, function wise.
Now with that said, I have done alot of research into both. I know how to field strip, properly lube and how they both function. I understand the 2 different rifles.
I would not hesitate to drag my Saiga through any mud, water, sand and so on. I would hesitate to do so with my SAI, only because it is a Walnut stock.
The AK has less moving parts, not machined as tightly and looser tolerances, which means it will feed with debris, function with mud and run with very little maintenance.
The SAI, standard is machined much tighter, but should function just fine.
Now the NM and Supers are machined even tighter then the standard. They were made to be super accurate for a cleaner environment.
For the money, the Saiga 308 will be around $500 new, then $200-300 to convert to pistol grip.
A 7.62X39 AK, range from $350-$700.
If you have the funds, and want a great, accurate and beautiful rifle, do the SAI. I suggest the standard in 21". The SOCOM has a different gas system then the Regular.