hey dave! you'll have a little more luck posting in the "strings" section, but to answer your question....
stainless steel strings are general very bright sounding.
nickels are a bit warmer. i use nickels and normally play with a pick, just my preference.
flats are very mellow and old school sounding. think jamerson or duck dunn.
there are also half wounds which i don't have any experience with, but are supposed to be a little mellower than flats.
this is just a very general answer, but i'm sure someone smarter than me will be along soon enough! best of luck
Fender jazz #904/P-bass #932/Never be a Venue that Charges ME to Play Club#12/GB Club #428/Sunn owner #79