You will spend the $500 savings + to get the Polytech up to snuff and in the end will have a forged receiver rifle.
That said, I have three investment cast M1A rifles, all work just fine but none are exactly current factory either.
Mine all have USGI bolts, trigger groups, USGI or quality aftermarket operating rods and sight assemblies.
The gas systems use a mix of USGI and high quality aftermarket parts also.
One is on its third Spring Inc. barrel, one has a USGI barrel and the Match rifle is on its fourth aftermarket barrel.
Your question is a pertinent one.
'Is a Spring Inc. better than a Polytech, (In factory form)?'
In my humble opinion, no, both are comparable and both require a bit of work and tweaking to get them up to the standards of a genuine military issue basic M14 rifle.
Your other two questions which is better, a stainless steel barrel or a walnut stock?
Does either make the rifle shoot better?
A walnut stock will not make the gun shoot better, it will make the rifle look better.
A stainless steel barrel may, or may not, make the weapon shoot better, that is entirely up to the quality standards of the barrel.
As a general rule Spring Inc. S.S. barrels do tend to produce better groups than the chrome moly barrels. as is right from the box.