its down to preference for each player, I personally prefer the seimitsus, lots prefer sanwa, we have been having this same discussion on killercabs forums and its a regularly debated topic on shoryuken.com hardware forums
IMVHO seimitsus are quieter, flow easier and smoother, take alot more hammer and the switches dont require to be changed as often as on sanwa's.
out of 5 of my cabs, 4 are fitted with seimitsus and one is having sanwas and seimitsus (swappable panel)
Rod @ Himura games stocks both sanwa and seimitsus