I have seen a few opinions expressed but I've not come across a technical read on this issue that would rise above one persons opinion or anecdot vs another.
My opinion (not backed by anything other than personal experience) is that on average, there is very little if any difference. The panel antenna that works in one application is beaten by a Yagi in another and vise versa. When one is down to differences of 1 or 2 dB and still has no solid lock on a signal, don't expect dramatic differences.
Ken Nist (hdtvprimer) tried building a
16 bay panel and even a
32 bay panel which would have plenty of aperture. his success or failure would not be the final word on the subject, but as I recall he was left to theorize that not all elements were 'illuminated' due to diffraction of the signal. If that theory was accurate, a single Yagi with a few dB
less gain may have performed just as well or better.
But if multipath is due to variable conditions (moving reflective surfaces, etc.) neither panel nor Yagi would seem inherently superior to the other.
With out a means to accurately measure the conditions you're left to experiment.
If the well is dry and you don't see rain on the horizon, you'll need to dig the hole deeper.