Kenth,
I have both and use them for some boxes, mainly offcenter work. Most of the time I prefer the backhollowing technique as it is faster. But the Key gouges have less side load and don't put as much stress on the tenon on longer pieces. I learned to use them watching the Ray Key videos and a lot of practice.
Initially you use the gouge as a spade bit, with the flute rolled about 5-10 degrees off vertical - flute facing a little past 9:00. This sets your depth, I have marks on my gouge for the standard depths I use.
Set the rest so the tool is at center and fairly close to the wood. With the flute rolled to 45 degrees, you pivot the tool on the rest from center hole back towards 9:00. I do rub the bevel on this cut. You will be swinging the tool through an arc as you go, this is not a "pull cut". You can also do the same thing with a 3/8" shallow gouge with a fingernail grind on it, using it the same way.
To finish the inside, you use a scraper or scrapers depending on the shape of the inside. Ray uses a long side scraper with a radius on the left hand corner and the end ground back to somewhere between 80-85 degrees. I do a lot of rounded bottoms, so I use a Raffan "box" scraper most of the time. The scraper and sandpaper is where the fine finish comes from, the gouge is just for removing wood quickly.
If you are doing small boxes or bowls the 3/8" will probably be easier for you to use. The 1/2" can take too large a cut in a small box.
Fog