What really is the difference?

I don't know what you mean by "Darwinian" fitness, as opposed to fitness as the word is used in ordinary evolutionary biology. People tack on the word Darwinian to all sorts of things and may even used the word disparagingly. Survival of the fittest is a phrase meant to suggest the process of selection. I personally avoid it because there are a lot of false ideas attached to it. Sorry I can't be of more help, but I don't really understand your question.
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What really is the difference?
My memory's a bit hazy, but I think it was a 'reviewer' who used the term 'survival of the fittest' to describe Darwin's ideas on origins and descent.