At one time I had the whole range from 331 to 881. I still have the 331, 771 and 881.
My favourite is my 771 though so much so that I'm even thinking of selling the 881 ( I have over 20 amps and need to thin down my collection!)
Here in Belgium, the smaller ones 331 441 and 551 are pretty cheap ( when they turn up) and represent great value for money, little money for really good sweet sounding amps. The 441 is pretty rare though, and strangely enough the same power as the 331, difficult to understand Sansui's marketing reason behind the 441.
The 331, 441 and 551 are cap coupled, the 331 uses a single stereo Sanyo STK IC chip, the 441 uses two STK chips I believe, whereas the 551 uses discreet transistors. The STK chips are practically unobtainium, but they seem to be fairly strong.The smaller ones all sound essentially similar.
I would get the 551 if available mainly because of the discreet components. Often the bulbs are blown (and may impact the sale price which is good as it's only a few minutes work to swap the 3 or 4 bulbs with modern LED bulbs.