What you buy can depend on the strings your bass was set up for. For example, my bass when I bought it was set up for mediium-lights. And it just so happens I like the med-light gauge. But if I were to switch to say mediums, I would have to adjust my bass in order to keep the strings properly tuned (I would take it to the shop personally). So gauge is a personal preference. I like medium-lights; it is a good all around setting. They are bright enough and still provide a rich tone on the B string. It's very balanced for me. Hope this helps;
God Bless! :wink: