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I strongly suggest you pick up a copy of the latest (or even an earlier) edition of the "Bosch Automotive Handbook", essentially an engineering manual that's written in a prose that laymen (such as myself) can understand and learn from.
Before answering your question, however, I would suggest that you also consider the e-code substitutes (i.e., H4 bulbs in a "sealed beam sized housing), especially the Cibie brand, for your choice, regardless of your final decision on style. See www.danielsternlighting.com for a selection of the best headlights (IMHO).
That said, I'll summarize Bosch by saying that the answer is that the round configuration is far superior to the rectangulars. And aside from the book answer, you can see that for yourself if you examine the reflectors in a round and comparable rectangular lamps. The round, parabolic reflector is complete (think of a hemisphere of a globe, though it's not quite that), surrounding the filament uniformly all around it -- as a result, it captures and reflects all of the photons (eminating from the point source filament) forward through the front lens. In contrast, the reflector in a rectangular lamp is (using the same analogy) a hemisphere of a globe that has been chopped off at each pole from about 60 deg north and south latitude to their poles -- as a result, it has less (for it's size) reflector surface, and therefore reflects fewer photons forward.
Bosch goes on to also add that the larger lamp (measured horizontally) will have more useful light output, so the old "dual" 7 inch rounds are more efficient than the smaller "quad" 5.5 inch rounds, but at least with the quads, you've got twice as many lamps than the old dual setup.
As a subjective example, I used to own, simultaneously, both an '80 (245DL) with the quad rounds and an '84 DL with the quad rectangulars -- both had the Cibie e-codes with identical H4's inside -- and the rounds still had a noticeably stronger illumination.
Hope this helped.
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