Which is less insecure - mounting /system rw or disabling SELinux
I'm not an expert, but disabling SELinux is almost certainly more insecure.

Nothing that's not already in /system should be able to write there without root in either case, or an exploit that would let malicious code remount it anyways.  But disabling SELinux hamstrings Android's process sandboxing and access controls, meaning a malicious app could modify or steal data from others.