You added some unique perspective from your

direct experiences working with both Wharton MBAs and underghraduates.  However, it would be very hard for me to imagine that Wharton MBAs or students with a STEM background will be sharper or smarter all around than Wharton undergraduates.  It is more likely from a selection bias and judging based on  experience with a single (particular type of) firm; it could be that the nature of your work may use some particular background or knowledge that only the STEM students have received the training or education.  Also, remember that for admission into Wharton undergraduate progarm, the student must have done exceptional well in math, in addition to be a overall strong achiever; therefore most of them have the intelligence doing well in whatever they decide to pursue.

As for your comment on the STEM PhD, this is way off ; you are trying to put the STEM PhDs in a way more optimistic position than the reality they are facing.  I can tell you STEM PhD students overall are struggling in landing good jobs, although some majors such as engineering may do pretty well.  You statement of "STEM PhD students don't even look at that (buyside) job" is laughable.  The reality is thing will rarely happen even they try to land buyside jobs, because the buyside analyst slots that require a STEM background are just limited, and it is certainly not applicable to the majority of funds or positions.  I personally know someone who did undergrad at Columbia and did PhD at Upenn; when that person was able to go MBB after getting PhD (for healthcare/pharmaceutical consulting; so a niche position), others were pretty envious and considered the gig as highly lucrative (starting at ~$200k/year all-in).