About the only significant differences are that the spin dryer will usually spin much faster than the salad spinner ...
Man I don't know I get this salad spinner spinning pretty darn fast . . . biggest problem is getting the load balanced. Usually only 3 socks at a time or a carefully rolled up and stuffed t-shirt. Guess I will have to check the crank to drum ratio and figure out how fast it is spinning.
My washing machine spin-drys pretty well already and I don't use heated dry (just blows are through tumbling clothes) for shrinkage related reasons.
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DracoFelis: Sorry, I should have been more detailed in my reply. In my situation, where I hang dry ALL of my clothes, a SPIN dryer would only cost me in electricity and time. I hang my stuff in the morning, and it is dry by the end of the day. I also get the benefit of receiving the sun's bacteria/mold killing abilities by hang-drying my clothes.
If you are using a standard washer and then moving your clothes to a normal clothes dryer, then it would definitely make sense to add a SPIN dryer into your laundry cycle. It most definitely would help reduce time in the electricity/gas powered dryer.
I like my setup, and for those who do their laundry in the same fashion as me, there exists little to no plusses for me to use a SPIN dryer. After all, the unit is obviously not free, and it does use electricity. My setup uses no power, and costs nothing, short of menial clothes hanger/clothes line costs.