If you are available on both the days, rather than pick one and (implicitly) reject the other, I suggest you respond with your preferred date as the 1st preference, and the other date as the 2nd preference. That way, it is less awkward for them to reschedule to the other date if they run into problems on the agreed date, while you don't lose anything.
A few years ago, I was asked to pick one of three dates for an interview (with those days being Monday, Thursday, and Friday). I picked the Monday, only to hear from them couple of days later, that Monday is "not possible", and I should pick either Thursday or Friday. I picked Thursday, and the next day, they called again to inform Thursday is also "not possible" and hence, interview is scheduled on Friday!
I was bemused by what I perceived as silliness at the time, and wondered why they even bothered giving me options in the first place. Later, with more experience, I could see it from their perspective, and could imagine how awkward they must have felt to make those calls. Listing those dates in the order of my preference would have made it less awkward for them.