Miles v. Kilometres

When I started this blog I was measuring my rides in kilometres, now I am measuring in miles. But I still cannot decide which to use. I started with kilometres as I was trying to get used to them as so many events are now in kilometres. But I then moved to miles as I found I was constantly converting to miles in my head to get an idea of how fast(slow) I was going and how far I had gone. Here in the UK all the road signs are in miles, my car measures in miles so I am used to miles. Then I went on my Trail Leader Award and I had to have my computer set to Kilometres and route planning was made easy by the fact the map is marked off in kilometre squares. So I am still confused, should I stick to miles or return to kilometres? Should I have an aim of riding 6000miles this year or 10000km? Will an average speed of 13.5 km/h make me feel faster than 8.5 mph. Am I just indecisive?