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I think Berkman's stock is higher (without being a significantly better player - close to a push, maybe 0-5 runs better assuming both are healthy) and hes also making more money.
I'd be ok with either as mid-season acquisition, I just wouldn't give up any actual talent for them (maybe a C prospect and both teams treating it as mostly a salary dump).
Trading Lopez now for Berkman would open up a hole at 2B until the '10 offseason.