Ranger,
The thickness will depend a lot on what your intended purpose is. If you are training with it in an Iai-do style, and don't intend to cut with it, go witht he 6mm. If however you are going to cut with it, go with the 8mm. Also, the extra width on a display peice will add to the impressive nature of the sword with out taking away from the elegance of it.
I forged several too thin before realizing the proper thickness for my katana. I trained in a kenjutsu style that demanded a long blade, and consequently more thickness was needed. We did some cutting on light targets and I was glad I made mine a little thicker.
Hope this helps,