The answer is: It depends. If your running AC or Legacy, without a doubt the Lionel/Flyer Pacific at $699.99 is the better value, even before discounting. You'd be hard pressed to find the AM Pacific at a discount.
However if your looking for Scale operation, the AM model is "it" as there are no scale wheel conversions for the Lionel/Flyer Pacifics.
Detail wise, the Lionel/Flyer Pacific wins hands down. It is a very good representation of a USRA light Pacific. This is the TMCC version from 2007:
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The AM Pacific is almost Mantua-like (HO) with its minimal detail. However, the AM model is a good "blank slate" for detailing if one so desires.
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That said, at $600 for the projected price for the (I assume conventional DC) AM model seems a little high. But I guess you have to put it in perspective, it's not 2008 anymore when the DC model sold for $300.. I don't think even Lionel would be able to maintain the current $699.99 asking price if they do a following run of Pacifics or Mikados.
And of course, AM doesn't offer a command version and the TMCC/Legacy sounds are far superior to the Ott (last I heard) sound board AM uses.