The circular columns would probably be preferred, because they can generally handle higher loads under a variety of adverse environmental conditions (high wind, earthquake, etc.), if they have a closely-spaced spiral reinforcing bar inside.
I'm writing an article on the strength of round columns verses square and came across your post. Would you like to contribute a "experts" quote to the article similar to your post? If so, please send your quote ASAP to firstname.lastname@example.org, along with your title or credentials. Thanks so much!
I'm no expert; that is in part why I asked the question. By the fact that circular columns have greater symmetry (evinced throughout mechanics), I would guess them strongest in general. Of course, if the building itself were square/rectangular (as most are), square/rectangular columns might reinforce its symmetry.
I have a Master's degree in physics from George Mason University, but like I say, I am no authority in this field. Don't construct a building on my word.