I don't think they are always better, some parts are made by the same company with the same materials which they supply both to MB and to the aftermarket. I think that there is more of a 'crap-shoot' attitude towards unknown parts as to warranty if something doesn't work. I know my tech only likes MB parts or parts from a (short) list of vendors, and I always check with him beforehand.
In the case of your springs, they may even have been made by the same company as makes springs for MB, but the specs may be different. From the previous discussion I referenced above, some companies make the same MB part but leave off the star and sell it as high quality aftermarket. Other companies may supply to MB, then cut corners on the parts they supply to the aftermarket, dumb as that seems.
That said, I only buy the parts I need from this site or the dealer, or accept what my trusted tech chooses. I don't have the resources to ferret out the good parts from the bad. BTW, real tacky for him to raise a stink in front of other customers!
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E