Yea, I prefer Seimitsu but could go either way with the buttons. The Sticks I strongly prefer Seimitsu. Not that Sanwa sticks are bad but Seimitsu feels heavier and clickier if that makes sense. Both are great, just my preference. Sanwa seems to be more popular though. Alot of premium sticks by default come with Sanwa sticks and buttons but alot of the older Japanese Candies come with Seimitsu.
I prefer the Hori Hayabusa buttons nowadays. Seimitsu is great for Fighting Games and Sanwa for Shmups. The Hayabusa buttons feel like a hybrid of Seimitsu & Sanwa. Like earlier mentioned try them out yourself, they are all great and there is no need to replace Hayabusa, Seimitsu or Sanwa buttons.
On buttons, it feels like Seimitsus have just a touch more resistance than Sanwa buttons. To me, it comes down to the look of the device the buttons are going in, to be honest.
On sticks, I think I actually prefer the Hori Hayabusa over any stick from Sanwa or Seimitsu, but you can't really go wrong with any of them. Stock Seimitsu LS-32/LS-32-1/LS-55/LS-56 springs are a bit stiffer than the stock Sanwa JLF or Hayabusa springs. I prefer a light spring for shooting games, but a heavier spring for everything else.
After using both buttons for a variety of installations. I have to say I like Semitsu more. They have a more crisp execution when playing with them. And imo their threaded screw on buttons are really nice to install.