I agree with the post #2 in that development and underdevelopment are not two separate policy choices a country can make. Development is, or can be, depending on a country's location, opportunities and resources, but underdevlopment is a condition - often chronic in nature - that many countries face without options to lift themselves out of it. Globalization was championed as a way for this to change, but it remains to be seen if it will be a long term benefit to the underveloped world.
I think that more specific detail in the question might not be a bad thing. It seems like you are trying to forge a rather interesting analysis here and trying to grasp where your thoughts are on this might be very distinctive. The previous post was quite thorough in the assessment that traditionally held concepts of development and underdevelopment are separate elements. In a more philosophical sense though, it might be interesting to see if there is a relationship between development and underdevelopment. From the point of view of capitalism, one could make the argument that both are convergent with one another in that such an economic system is predicated upon a distinct individual who "wins" or who is successful and this very well could come at the cost of another, one who loses. The development of one group of people might be coexistent with the underdevelopment of another. From a more philosophical point of view, there are several schools of thought that argue creation and destruction, progress and devolution, development and underdevelopment are expressions of the same entity of consciousness. Finally, in Le Guin's short story, "The Ones Who Walk Away from Omelas," the coexistent and dependent relationship between development and underdevelopment is brought out in a very stark and intense manner. If the question is meant in a direct manner, then they are distinctly different from one another. If the question is meant in an explorative type of manner that delves beyond the surface, then perhaps one can see similarities and convergent nuances between both experiences.
I will not call development and underdevelopment as two sides of the same coin. Development refers to a process of growth or improvement, while underdevelopment refers to a stage or condition achieved during this process.
A country which has not been able to achieve much economic development in the past, but has just embarked on an effective programme of development may be develop very fast and become fairly well developed some time in future. But for the present it will still be a an underdeveloped country. Similarly a highly developed country may currently experience a very low rate of further development.