I grew up playing on American parts, so that’s what I prefer.
As far as actual buttons, I prefer the concave with the horizontal switch (same idea as competitions, just the button is concave). The Ultimates themselves (which is what MAS uses for concaves) are a different design completely, and I can’t recommend them (they have the switch vertical, and pressing the button moves a lever to hit the switch. Over time, this can break, and I’ve actually seen it happen when you really don’t want it to).
On the stick, I’m good with either the Super OR a Competition. The Super works on the same leaf principle as the Ultimate buttons, but these are metal leaves on the switches themselves. This seems odd at first, but it makes sense in practice.
The Competitions use an extra piece on the bottom to hit the switches directly, but in my experience, it seems slightly tougher to get only a single direction. Of course, Happ doesn’t actually MAKE their own parts, and they’ve changed manufacturers fairly recently. The Competition’s quality kinda suffered in the changeover, but I’m not so sure about the Super.
(Note: my Super impressions come from the chinese knockoff found in the stock SFAC stick)
Layouts, again, I prefer American because that’s what I grew up on.
I’ve tried japanese parts and layout, but I didn’t like the way it all felt and the fact I could hardly play with it.
As I always say with this question, it’s a matter of personal preference.