25k Continental miles per year can be moved (1:1, no devaluation) to United by first transferring into Amtrak.
Of the airlines you list, only Continental participates in Hilton's
reward exchange.
United miles can be moved to another program via Diners Club (generally with a 50% devaluation).
But the first question is -- what are your reward goals? That'll dictate what miles you want to accumulate.
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View from the Wing: A blog about Free Miles and Free Markets