The Investigator is a great pen at a great price. I was into pens before I got into knives, and this pen offers exceptional quality and value. There are so many much more expensive pens that don't use anywhere near the quality, long lasting and tough materials that the Investigator does. It is as close to a "no-brainer" as it gets, IMO.
As for the "cool", fiddle, admire and play with it factor, I mess with mine a lot. And I don't seem to tire of it. I have owned the aluminum and the stainless steel, and I prefer the heft and quality feel of the stainless much more. I imagine that the brass and copper ones would feel much the same. If lighter weight is a factor for you, you would probably prefer the aluminum.
Good luck with your decision, and let us know what you decide. For what it is worth, I vote Investigator.