I don't know how good the viewing would be out of typical airplane windows. The sun is going to fairly high up (anywhere over the US), and might get distorted through the double pane glass typically used on airplanes because of the angle. But how easy is it to rent a plane with panoramic overhead windows (like some trains have), given how rare the use of them in an airplane would be?

If you're going to rent an airplane, you want to do something which no one in staying on land anywhere can do: Catch the total eclipse within a few minutes after sunrise or within a few minutes before sunset, when it will be near horizon (and the only time it's fairly safe to glance at the transition between totality and partiality without eye protection). The issue is that both of those events will happen way out in the middle of the ocean far from anything, so only a boat or a plane would provide access to them.