Depends on what you're showing. I find that doves look better U/8, when they are young they are a lovely clear bluey grey, but when they are adults the colour can become quite uneven and the does in particular can go saggy. This is probably true for all of the pale selfs. If you were showing red, fawn or sable, you're better off showing them U/8 because they have a tendency to get very fat and won't win anything, and because you need to breed these varieties when they are younger mice they have a very short showing career.
If you were showing marked mice (for example) you can show them for much longer. I believe agoutis look better as adults as this is when the fire comes into the coat. AOVs like agouti, cinnamon and argente that are too pale to show as youngsters will probably make great showing adults because these varieties all darken with age.
Sarah xxx