The choice of strings can greatly affect a bass' tone. In general, gut or gut core strings have the deepest tone, but they are not always the easiest to bow. Also, they may lack the sustain that steel strings can produce for Jazz pizzicato.
Steel, on the other hand, is usually better for bowing and for pizz sustain, but can lack the punch and fundamental that gut produces.
Of course, there's many variations on this generalization. There's deeper, darker sounding steel strings and there's brighter ones. Some bow better than others. There are some guts that have more sustain than others, and some gut that's very pleasant to bow on. Also, there are many new synthetic core strings that can have some of the characteristics of both.
Finally, different basses respond differently to strings. What sounds great on one may not on another. It takes a lot of experimenting to find the best string for your needs. And, as anyone on this forum will tell you, that experimenting costs mucho money!
