I have heard that the reason that many siddurim print משיב הרוח ומוריד הגשם with a segol is because it is more common for גשם with an etnachta to be vocalised that way, rather than with a kamatz.
Is there any statistical analysis on the commonality of one vocalisation vs. the other in Tanach when accented with an etnachta? In other words, which form is more common?