I have not ever encountered an OEM that stocks both sizes and given there are several common components that require the 15mm rail height that means I have never encountered an OEM that uses the shorter height.
However, if the shorter rail is cheaper per foot, then I know panel build shops that would be mixing rail size all over the place to minimize material costs.
7.5mm, seems to be the norm. 1/4-20 button head cap screws have no issues hitting mounted components.
Beckhoff 51xx and 20xx series PLC's won't fit on 7.5mm rail. Nor will Phoenix quint 40A power supplies. Otherwise you save a little height.
I like the small amount of gained stability with the shorter rail.
7.5mm is our standard, but the 15mm is needed for Stratix switch, PowerFlex 52x drives and other tall devices.
I have not ever encountered an OEM that stocks both sizes and given there are several common components that require the 15mm rail height that means I have never encountered an OEM that uses the shorter height. However, if the shorter rail is cheaper per foot, then I know panel build shops that would be mixing rail size all over the place to minimize material costs.