I've got a SA90/8 and an SWD90/5.6.
Hands down, the Fujinon SWD is sharper. But then, you asked to compare a SW not an SWD, so I can't help you there. A good SWD might set you back $300, so unless money is ridiculously tight I'd recommend one of them anyway. Takes 82mm filters and is huge, so size might be another reason to stick with the smaller ones.
The Seiko shutters are a bit interesting. Not on my SWD90, but on my old-W 210mm. You can't open the shutter for preview/focussing unless it's cocked first. Also, if it's open, you can press the shutter at which point it will close, then open/close to take the shot. Not a massive deal, but just different to all my other shutters.