I researched this a lot when I purchased some new upper and lower arms for my car. There are in fact two different designs. Most things you will see will state that the AMG and the Sport version Mercedes W202's had "reinforced upper control arms" that went with the Code 956 sport package. The arms themselves are NOT reinforced. The rubber bushings used in standard and the Code 956 arms ARE different. The 956 arms are stiffer.
What I confirmed though is most of the reputable after market parts manufacturers all make their replacement arms now with the stiffer code956 quality bushings. You can find the inferior softer bushings but those arms are a lot more rare AND are more expensive.
I will also note for my chassis dynamics expertise that with long spindle suspensions like these car do not exert a lot of force on the upper control arms as exerted on the lower control arm bushings. The bushing when new really will feel pretty much identical, its just the stiffer ones will hold up longer to abuse over years of use.