I think your 18 watt LED bulb in a HPS (or MH) fixture will produce a lot less light.
HPS and LED produce about the same number of lumens per watt. To get the same amount of light from an LED as a 70 watt HPS, the same amount of heat is created. An LED can't survive that in an enclosed fixture.
The LED is different because it's very directional. This allows you to use a fixture that puts all the light where it's needed and almost none where it isn't needed. The reduction in waste means fewer lumens and watts are needed.
A fixture made for an LED will have heat sinks and might aim the light produced in one direction. Depending on how much light spread you need, you might choose an LED or HPS.
