Good question, guilotine, and I agree with you, one should always be aware that mainstream religion tends to be more about propaganda created to glorify their teacher and impress the masses.
On the other hand, I came away from reading the suttas through several times that, in addition to both of these disciples most importantly being free of the fetters, Sariputha was more about critical thinking, and possibly an excellent memory for the teachings of Siddhartha Gautama; whereas, Muggalana seemed to be more about determination and rigorous practice. I see that all of these skills are requisites for genuine attainment.
Siddhartha Gautama was more about having been of course free of the fetters, but also had the ability to pull together the most cogent and logically true philosophical system on record, as well as the clearest and most concise description of the mystical experience on record. This means he most assuredly negotiated the entire package to fruition; whereas, both Sariputha and Muggalana just got free of the fetters, by most probably mastering just the 4 jhanas; whereas, Siddhartha Gautama most assuredly mastered all 8 stages of samadhi, which does have added advantages.
So, what I got out of the suttas is, arahat is free of the fetters, but Buddhahood has full fruition and mastery of the superior fruit.