i like my tanks to look natural, so a whilte/ brown substrate would go in my tank. but this is your tank though...
black usually makes fish colour up, and white can make fish look bleached out. hence the brown substrate...
the mantis will be happy either way. as long as it can burrow it wont care about colour. only thing is, white sand will reflect light so your tank will look brighter. so your mantis may go shy for a bit.