Here in NW Oregon we are the nursery capital of America for the most part. Millions of mugos are grown here as well as millions of JBP. Mugos grow slow compared to JBP, anywhere you are this is the way it is. By nature, a mugo will put on 6" of growth in a year if you are lucky, whereas the JBP will put on several feet in one year. This is with both trees in the ground and established.
If I had to choose a species of the 2 to play with it would depend on where I lived. If I lived in Japan I would play with JBP, if I lived in Europe I would be playing with yamadori mugo and since I live in America I will play with Ponderosa, Lodgepole and field grown JBP and Scots.
Both trees will reduce needles nicely and get really small. By nature mugo has a smaller needle but you can reduce JBP needles much further then mugo and almost get jbp needles as small as mugo needles. It is all based on technique. The difference between the 2 needle lengths is not enough to make a big deal over, really. I have seen JBP needles reduce smaller then Mugo just like Chris mentions.
At the end of the day, Europe has the best mugos and Japan has the best JBP. Nursery Mugos, or American mugos dont' even begin to compare with mugos in Europe..... Then there is the bark comparisons...again JBP smokes the mugho here......
Jason