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Thanks for the advice so far... from two answers it's still 50/50! Both very constructive so thanks very much indeed... more opinions would be appreciated.
My heart says keep the 600 as I really enjoy driving it, but my head is saying the 75 as it's newer and should be more reliable.
Sounds like a luxurious position to be in, but it's not (I bought the 75 as the 600 had a problem with the injector pump which has sorted itself out so I now don't need two at all!) and I could do with some cash quick :o)