These are some real basic questions, but I would prefer to do it right and avoid potential isssues later.

For installation of new light switches and additional outlets...

Are metal or plastic boxes better? Why?

The boxed have a 3/8" offset marker on them, but I am using 1/2" sheetrock. Does it matter if I use the marker and have the front edge of the box recessed 1/8" behind the face of sheetrock"

Boxes for low voltage are orange plastic with no back to the box. Is there any reason to use these boxes instead of regular blue plastic electric boxes that are one fifth the price?

If you are using Romex, then use plastic boxes.

Does not matter whether the box has a mark on it or not. The front edge of the box must be flush with the finish surface. Use a piece of scrap material to set the box exactly the dimension that brings it flush.

No,you cannot use the data low voltage rings for line voltage devices. All line voltage devices ans splices must be completely enclosed in an approved box with cover.

If you are using Romex, then use plastic boxes.

Does not matter whether the box has a mark on it or not. The front edge of the box must be flush with the finish surface. Use a piece of scrap material to set the box exactly the dimension that brings it flush.

No,you cannot use the data low voltage rings for line voltage devices. All line voltage devices ans splices must be completely enclosed in an approved box with cover.

Sorry for the miscommunication. I was contemplating using the orange boxes for low voltage (phone/data) and wanted to know if there is a good reason to use these boxes instead of the blue plastic for low voltage as the orange boxes are 5 times the price. Do they for example provide insulation for line noise?

Nothing plastic offers noise reduction. The twisted pairs of data cables helps eliminate some types of interference. Don't run data cables closer than 2 inches from power cables to keep away from the magnetic field of AC cables. Each should cross only at 90 degrees.

An enlosed box for data cable only wastes more money.