(This became more a longer comment as I asked myself recently too, what are the dis-/advantages of reddit, Quora, SE for scientific questions and discussions)
Why not use Quora, Reddit and SE for different topics or even crosspost your questions, copypaste cannot be a time factor?!
As mad scientist stated, SE has another system and format than Quora. Sites need to grow in an organic way, this is a advantage and disadvantage (not every topic available as on quora). Quora will never reach the amount and level of high quality content sites like mathematica.se, computationalscience.se, physics.se, cstheory.se create. For humanities and social sciences Quora has imho a higher level, as SE started as a progamming site. Its hard to attract experts of those branches to area51, so often diluted by interested laymen if the proposal is popular and the final quality will be limited and more on a popular science level.
Surely, there are biology questions where you will currently receive better anwers on Quora. But when it comes to bioinformatics, biophysics, biochemistry I think you will find within the next 1-2 years the better Q&A's here, as here is more competition, commenting and voting and SE better suited for students which often come up with interesting questions during their thesis. This system is a better representation of a real scientific community than the follow and trust your expert on Quora, which lacks discussion too much. Starting proposals on archeology, geology, anthropology however seems hard on SE currently.
I think the much smaller attention on comments and upvoting comments on specific answers and questions doesn't fit science topics in a good way on Quora. Many science boards and forums on the internet are still pure discussion plattforms, open-ended, quoting of other user. Many scientific problems and questions simply fit a discussion like publishing and Q&A system (SE) better than a pure ask one expert system. Sciene is discussion, its not a matter of trusting an expert and the amount of omniscient high-followed full-time users doesn't further my scientific skepticism on Quora, too much "networking" of experts seeing who votes whom, voting should happen anonymously. Many very localized questions on Quora have only one answer, few comments/voting.
The voting on Quora seems to be much more a matter of master/phd grade next to name than on SE (I think its much better to read a answer at first and afterwards taking a look at the profile). Thats how I read a paper. Contrary, the voting for putting effort in an answer will often drive an expert to answer on SE, on Quora there is not really an incentive for answering localized questions, mostly popular ones.
Frankly, I have to say that most questions on Quora remind me more on those popular science one liners on reddit, its too hard to recognize interesting but localized questions, on SE those will be voted by some regulars. The answers are often longer and better referenced on Quora, but often google and wikipedia stuff simply replicated. I dont see Quora creating content like SE sites mentioned above when it comes to practical problems and questions.
To understand science and get a feet in a distinct scientific topic, it are mostly less the popular one-liner questions or long wikipedia like answers but the link and understanding of specific different questions with clear context bothering current scientists and students that will really help you to progress with your scientific work or learning. Asking good clear questions is IMHO more important than reading loads of books and answers and often gives you the option to answer your question on your own. And not being able to vote/close questions on quora/reddit is the biggest disadvantage of those sites. You don't learn asking nor will read many good questions. Its hard to find localized questions related to current research, besides all this popular sciene stuff. Sorting questions on SE via voting or number of answers gives you a easy overview what problems bother current students and professionals in research and the state of knowledge is.
So, in your case, I would crosspost your questions to Quora and SE currently. but in 2-3 years IMHO Quora will be behind SE in all STEM fields and I also believe the quality on Quora will fall with time like on reddit. I dont see how Quora or reddit can manage the signal/noise problem without serious moderation. On SE, you can still create a theoretical biology, bioinformatics, synthetic biology site... if there is enough interest.
