Tate! Welcome back!
iirc you bought a SSR56 for your son. This would take the 7.62 waffles. There are pros and cons on plastic vs steel.
1st off on 7.62 you should have no insertion, or feeding problems. If anything waffles will fit a bit tighter in the magwell(less slop). The only AK mags(M43),I have ever had a problem with was a steel one, that had a small ding(really small)in the side. Being steel this was easy to fix. AK mags are from a completely different school of thought than AR mags. AR mags are basically disposable, they have a limited intended life span. AK mags on the otherhand are designed to have a service life almost as long as the rifle. You can keep AK mags loaded with no effect, unlike AR mags. If you look closely at the pictures coming out of Afghanastan, you will see some 1st generation Russian mags still being used in very rough conditions.
Waffles should run you about $3~4 more/ea. (see:
www.joeken.com/akmags.htm ) The only downside I see is I don't know what the long term durability on the plastic is. I mean for casual shooting, you should be fine. I am talking about 10~15 years of throwing the mag around in rough service, in hot and cold extremes(I know Valmet uses this design), will the polymers take this abuse.
I guess it is a matter of personal preference, I have a large number of both, but tend to shoot the metal ones more.