Thanks for the detailed replies guys.
I was sort of dabbling with the idea of the 300 win mag for doing some real long range stuff from time to time, but it would only be for fun and a bit of mucking about occasionally. If I could do that and get some benefit inside my normal 100-500m range then I might have gone with it.
Not smart to go with it though for some fun 1% of the time while making it more expensive to shoot and more difficult the other 99% of the time.
I am going to go with .308