After extensive testing using an FM CP that was not yet on the air officially, I decided on vertical only for all my 5 FMs.
The city and area is very "hilly". So we tried a variety of single bay antennas that could be rotated 90° to be either vertical or horizontal in just a few moments after a short climb up the tower. The location was inside an urban area of about 1.2 million people, and near the geographic center. It was about 300 meters above the true center of the city.
We made a list of a couple of dozen sites we could drive to in a single day, and did two total runs... vertical with a vertical receiving antenna, vertical with a horizontal receiving antenna, horizontal with both vertical and horizontal antennas.
We used a simple read off the pre-AGC of a car radio as well as an evaluation of the audio arriving and leaving each monitor point.
The overwhelming conclusion was that our market and terrain was vastly improved with just vertical.
A year or so later, we built a circularly polarized antenna, and did the same thing... but late at night as the station was then operating commercially. We found that adding "horizontal" did not help, and in the hilliest area where signals reflected from several "hills" it was worse.
(The highest of the "hills" was just over 15,600 feet AMSL)