Why decide? Why not use both?
View attachment 567163
I've rigged my bike up to take two identical trip computers when desired .... the one on the left set to Km and the one on the right set to miles.
When following Audax UK route sheet instructions I keep a very close eye on the Kms to make sure I turn in all the right places. I also set that one up when riding in Europe. I'm not blessed with Carol Vorderman's maths skills, so converting on the hoof isn't always a piece of cake, and without the miles display, I would seldom know exactly what speed I am doing in mph. Watching a Km speedo lulls you into thinking you are going faster than you are, and I find I need the speed in mph to keep to my customary pace on a long ride and make it back before dark.
I used to be indecisive ...