You are literally asking "Cardiologist or a Doctor , who earns more?"
BI Devs (good ones) are still software devs , they just specialised.....i'm sure if a software dev specialised in any language/product that is in demand right now the same apply...
Also "BI Dev" can be vastly different per technology, if you are a SAP BW Dev (A "BI Dev" on SAP) , you are probably rolling in the cash, while if you are a Microsoft BI Dev, you are most likely earning the same as a senior C# dev.
Same applies with "Software Dev" , Java vs C# vs Web vs Mobile vs Database vs Front End vs Senior vs Junior ...all different salary brackets.