It all depends on the school and the letter whether it holds any weight. Why would a science professor letter hold any more weight? Odds are, you spent a lot less time with that person than a doctor you shadowed. I have spent well over 60+ hours in the hospital with several docs...some more. They quizzed me, showed me how to do procedures and some even sent me home with homework. They have infinitely more beneficial input on whether I have what it takes to be a doctor over some PhD in the sciences. I know people like to think that being a good undergrad student equates to being a good med student and a good doctor, but that is complete crap. In fact, so far the people that have shown the best affinity towards medicine are the ones that aren't at the top of their class. Those people come off as abrasive neurotic headhunters oftentimes.
I'm just going to give ONE of my many examples with this. My dad was not the greatest of undergrad students. He wasn't bad, but he wasn't stellar. He got a D- in calculus and graduated with around a 3.1 (this was in the late 1950s before huge grade inflation). He went on to medical school and graduated 3rd in his class, AOA and all sorts of other awards. Did his prestigious residency and fellowship and wound up on staff for a medical school for a while. He was pushed by a physician from his hometown that gave him the same motivational type speech...he was having people spit the same crap about him not being smart enough. He was also having a lot of crap talked because of his religious and the fact he was in the deep south, but that is another story. You don't need to be a super genius. Just have to have a solid work ethic. That being said, for all of his talents, he admitted the best doctor in his office was the DO sitting next to him who also founded the practice. So, do well, apply to whatever schools in the region you want (ignore the letters after the name..they mean jack) and hang in the trenches with the people that are the best indicators. If the best indicator is that science prof then so be in. I have no idea why I went on this tangent...early drinking night I think