I do have a question: what is the difference between a Sotar and an equivalent Iwata?
There is a whole gob of stuff that no matter how well it gets covered, will wind up coming down to personal preference - so, it is a bit subjective.
The Sotar is probably my favorite Badger. I purchased my first Sotar mid 90's, and was really impressed with the brush then. I have owned several of them, but find that I actually like the original one the best. Fortunately, I was able to pick up an as new original a few months back. The angle of the color cup, as well as several other details have changed over the year - the original is a wonderfully machined piece of craftsmanship - the revisions not quite the same quality (there is something like 23 or 24 identifiable differences in machining). The most recent generation seems to be showing a return to a higher standard in machining quality, but I still prefer the original. It has better balance, and a smoother over all feel. The head assembly is where the business takes place, and the Sotar pieces do work well (I have maybe a few other Badger configurations, like model 100's, that I also have Sotar heads on). In comparison to a Japanese brush, I find that the spray pattern remains a bit more grainy, and I usually have to work at a higher pressure with the Sotar. There are times the Sotar will produce some silly fine lines, but it will tend to frustrate me when I really WANT it to. The trigger and needle action also seem almost gritty and stiff compared to a Japanese alternative.
There is not really a brush that is a direct comparison to the Sotar, but I will compare it to an Iwata HP-B. Right off the bat, the Iwata is about twice as expensive as the Badger. For me, there is no comparison in quality, with the machine work, fit and finish all going to the Japanese brush. I find the Iwata to simply be more consistent than the Badger. I can get it to operate at a lower air pressure, which will usually allow me to have more control. The action feels butter smooth on the Iwata, and once familiar with it, will allow some further fine tuning which turns them into gems to use. use my Badgers, but when the money is one line, I will reach for one of my HP-B's 90% of the time - the other 10% will get divided up between several Microns, and some German brushes I also enjoy using.
Both are capable and competent brushes. This is also just my take - the next guy in line may prefer the Sotar over any Iwata, or perhaps an H&S. The small little differences that feed into personal preferences are really what make the difference between a good brush, and a great brush. Unfortunately, that can be very different between any two given individuals.