I don't have the experience to know for sure if MH increases trichome production or has no effect one way or the other. What I have noticed is the white light makes it look like the buds are frostier. LED lights do the same thing. I'm set up right now for a test with super lemon haze. One group with a mixed light the other with straight hps. So I'll let you all know in a week or so.
As an aside I don't like to work under MHs and turn them off when I'm in the room and always use didymium safety glasses. The other thing I've noticed since I use open hoods, the MHs produce a lot more heat even if less light - a cooling problem for my setup.