I am with strings like my lovely wife is with shoes and handbags, and most of the double bass players I know are the same (see Clarky above)...The problem is that there is no consensus of opinion, and that every bass sounds different with any given set of strings. It is an absolute minefield (for the wallet too!). Clarky gave you an excellent summary there. My opinion is the following. If you love the growly modern jazz tone of lounge jazz trios (from 1960s onwards) get Spirocore (unless you are primarily an orchestral player). If you love the old school sound of the double bass, dark, dirty, percussive, then you need guts. Problem is, guts are expensive, don't stay in tune, are difficult to amplify faithfully and they are a general maintenance headache. So, there are lots of guts substitutes around. I have tried maybe not all of them, but lots of them. The good news is that some of them sound half decent, especially for live gigs. The bad news is that you will quickly realise that nothing comes close to guts.