In GENERAL (but not always the case), if a person has had ortho (braces) prior to 'make the bite right', the surgeon might need them to have the bite made' WRONG'(via a period on braces or tooth extraction)in the event they pursue maxfax surgery later down the line. Making a bite 'wrong' that was once made 'right' via prior orthodonture is called; DECOMPENSATION.
That's the very basic concept (which I hope I made simple to understand here) behind why someone with his 'bite made right' (prior via ortho) would be required to have transitional changes to his bite in preparation to have a maxfax (or bimax). One surgeon might elect to decompensate one way. Yet another surgeon might elect to decompensate another way.
Gunson's suggestion to you would resolve to what he would need to do to maximize the advancement of your lower jaw but to ALSO minimize unfavorable changes to the nose base that could arise by moving the upper jaw forward IN ORDER to maximize advancement to the lower jaw. Gunson also likes to to maximize lower jaw advancement so he can limit the extent he has to move the chin. He has a preference to move the chin point out via the lower jaw advancement so that dependence on the genio alone to do it is minimized. Basically, his chin advancements are not that much because he does some pretty 'elaborate' surgery to both jaws in accordance to his preferred aesthetic.
Assuming that you were seeking Gunson out for the same reason others seek him out which is BECAUSE you have ALSO familiarized yourself with his preferred aesthetic AND want that for yourself, he's informed you of what the contingency would be in order to have surgery with HIM. Hence, in order to have the type of aesthetic people SEEK him out for, he will need you to have the type of DECOMPENSATION he told you would be needed in your case.
Sinn telling you that you didn't need extractions reflects what I mentioned prior which is that surgeons may DIFFER in what they require for DECOMPENSATION which in turn is in accordance with a surgeons preferred aesthetic. So, BOTH can advance out both of your jaws for more overall projection and have different decompensation requirements to do so. It depends on their preferred aesthetic and also your OWN preference for one doctor's aesthetic over the other.
Sinn is telling you that your chin is recessive (YES,true it is) and he can advance it independant of any bimax surgery and of course, that won't require any decompensation prior change your bite in preparation for it. He also sees that BOTH jaws could be advanced (YES, true that is too.) without tooth extraction. It's certainly possible to advance both jaws EQUALLY (which is called 'linear advancement') and with that NOT change the bite at all before hand (and certainly not extract teeth to do it). BUT there is a LIMIT to how much to advance that way where unfavorable changes to the nose base would be the limitations.
Conclusion:
Both doctors could improve your aesthetic. Of the two, Gunson is associated with MAXIMIZING one's aesthetic for patients who might not 'need' all the surgery he does to do it and could 'get away' with still having improvements via LESS 'elaborate' surgery. Sinn is giving you the option to 'get away' with less surgery and/or FEWER contingencies than is Gunson.
So, WHO'S aesthetic outcome is 'best' and which option to choose, can't be answered FOR you. You would need to to at least see enough aesthetic outcomes of MANY patients of BOTH doctors and enough so to align a PERSONAL PREFERENCE for the type of outcomes EACH doctor kicks up so YOU could THINK about this more. What I can tell you is what I've already said here which was to explain why suggestions from different doctors can DIFFER.