Happ's sticks are also imported. Most of them were produced by iL (Industrias Lorenzo) in their facility in Spain. That is why they are referred to as Happ/iL sticks. The specific joystick used in the Street Fighter series kits cost quite a bit more than most Japanese joysticks did (more than twice as much), so this was obviously not a cost based decision.
I know that iL is a Spanish company and that they manufactured for Happ at some point, that's why buying an iL stick is now the way to get the quality of stick that Happ used to sell before they outsourced production to China. However, for whoever was assembling the cabinets, buying something from an American company that outsources production is certainly different than importing from a foreign company. To some extent this is semantics, but you called me out on the semantics.
I don't think I believe that Happ's iL produced sticks were more expensive than Japanese ones without seeing some evidence. Japanese sticks are about twice as expensive iL sticks are right now. I wouldn't think iL sticks would be more expensive when they had an agreement with an American company, so why would this be? Exchange rates?
The frail japanese controls wouldn't have held up in an american gaming environment. Americans didn't show much respect to things like arcade games and would break the joystick handles right off. I can't tell you the number of classic era imported japanese cabinets (mostly cocktails and minis) I have encountered with broken sticks and buttons.
I totally agree that this was part of the decision though.
I wouldn't go as far as calling them "frail," Japanese parts don't break down more if they're not misused; but like you said, it's a cultural thing, they were misused much more here. Agreed, they're not as durable in that environment.
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As for the original question, since the original poster asked specifically about the best stick to play Street Fighter I'll put it like this: if you want a stick that will best allow you excel specifically in fighting games (and the stick WILL make a huge difference), get a Sanwa JLF. It's the standard for serious competitive players. If anyone tells you anything other than a Sanwa JLF or a Seimitsu, they are most likely not serious about competing in fighting games.
Stop setting up your panel for that Mag-Stick. You don't have to get rid of it, it's good to have a parts bin with all kinds of things in it to try for different types of games. You can route panel so that it will accept both a JLF and a Seimitsu. Not hard, I can scan and post the mounting hole diagrams if you need them.
Get Sanwa buttons as well. They're a little more smooth and sensitive than the Seimitsus and way better for fighting games than American-style buttons.
Set everything up with a good button layout, and mount the stick so it the bottom of the ball top is about 23-24mm above the top of the control panel.