Well you could argue that because a failed nation only requires a simple military solution to correct, then that is easy for the United States to fix. But if the Nation is stable, then it is not as simple as sending some troops over. There are sovereignty issues, U.N. issues, opposing armies, etc etc. Also, any nation that is well organized poses a greater threat to the united states because it cannot be influenced as easily by us. Where as a failed state doesn’t even really effect us unless it involves our oil supply, in which case military solution is simple.
But personally I think failed states pose a bigger risk because they are more likely to get the united states tangled up in a mess we don’t want to be in militarily. Difficult to say for sure because the risks and situations are always different.