The interviewer should be familiar with biology, since those are the scientific issues that will dominate the next 4 years (and many years beyond). The budget for NIH alone makes such an exploration essential. Collins is one of those rare individuals who happily sees science and religion coexisting. He can help lay to rest the tired myth that science is at war with religion, which seems pretty important since many citizens increasingly equate science with atheism..
Francis is an interesting and sometimes entertaining scientist, but as Director of NIH, he is also a member of the Obama administration. If you were the opposition party, would you accept him as moderator? I want to see this debate happen, not let one party cry foul over the appearance of a biased format.