My understanding is to always switch positive.
That way if there is a short to ground some where in the system, current will not flow.
That is why household current always switches the black wire (positive in DC talk) and not the neutral (negative in DC talk).
Here is my setup:
I was almost finished in this photo and the black connector wire between batteries was not finished. I used two sink dish wash tubs for the battery box.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW