Fixed wing first then rotary.
Flying isn't just about the aircraft it's also about the environment, safety, RT, flight planning, navigation etc. etc.
If you are really planning on getting both then learning the 'flying' bit is easier in a fixed wing. Leaves you a lot more time to get your head around the other important stuff.
Once you are comfortable with that you can progress to rotary wing.
(like pulling the nose down when the want to lower the flight hight etc.)
Just for info, once past the transition (from hover to forward flight) pushing the nose of a helicopter down will also 'lower the flight height', whilst increasing airspeed. It's an aircraft after all. I think you are referring to the effect of lowering the collective which will reduce you 'height' but not increase your airspeed. A helicopter will 'dive' as well as a fixed wing.
You don't need to 'unteach' yourself you just need to be aware of the different techniques that need to be applied to both.