Pixels/inch OR Pixels/cm

It does not matter which. If you are used to working in inches, just stay with that.
What is important is the resolution and size (in inches or centimeters) that you set up the file with at the beginning of the process, and it looks like you are doing that correctly.

Ok Great. So there is really no quality difference between pixels per inch or pixels per cm. Just like building in inchs rather than pixels.

Thank you for your help.

However, how do they calculate that. I would think ( at 300 dpi), 300 pixels/in would have a lot less pixels than 300 pixels/cm

I do not know the conversions well enough to figure that 15" equals "X" in centimeters.... All I know is that I will work in inches because that is what I know, and I will create my original document at the (or larger) that it will print, at a resolution of 300 PPI and I am good to go. If you wanted to work in Centimeters you would have to know the number of centimeters the finished print will be and create your document from there.

Or better yet, I stick with vector art whenever it makes sense for the design....