1. The glass is for protection from excessive UV rays. You have to decide if YOU need that protection. It can be harmful to the upper buds. I had to offset my plants some, to keep them from getting 'sunburn'. The reflector plays a big part, here. Not all reflectors are created equal.

2. The glass is useful for holding heat away from the plants...provided you are using an air cooled hood. Resin/trich production is more genetic, than light usage...IMO.

3. I've done grows with only HPS, and only with (C)MH....same strain, no major difference...except (C)MH plants are about a foot shorter....and a little thicker.

4. (C)MH - Ceramic Metal Halide