Dave wrote:
A well grounded mast DOES NOT attract lightning any
better than a non grounded mast at the same location.

actually it does. both the ieee and cigre have been using lightning
statistics data collected mostly from tall masts for many years.
There are well known formulas used to calculate the number of strokes
to a pole or power line, both include the height, and as height
increases so does the number of strokes to the object. The height
also skews the current distribution with higher structures more likely
to get more high current strokes.

But, is there a difference in strike rate between grounded and
ungrounded towers of the same height. I would think that the difference
would be very small, and smaller as the height gets bigger.

Since the vast majority of commercial masts,towers,buildings used to
collect the stats are probably grounded (Because the code requires
it...), it might be hard to find decent data for "ungrounded" things.
(for one thing, the equipment used to collect the strike data, until
recently, probably measured the current spike on the grounding wire..
these days, you could use the RF lightning detection systems, and match
up strike locations against structure locations)

Maybe wooden poles? (which are only "sort of grounded")