Really doesn't matter from a technical standpoint. Use the Slug from one and the screw coil of the other. Having the hotter coil closer to the bridge or neck would make a bit of difference tonallu of course.
Custom Slug side if you're going to split the pickup because it gives you around 7k...I always use the Custom slug bobbin paired to either a PGB or 59B screw bobbin.. :wink: