With about 66 million people, the Democratic Republic of the Congo (formerly Zaire), is endowed with a vast potential wealth of resources, but has been plagued by wars, corruption, mismanagement, and poverty. Annual GDP growth has been between 6% and 7% over the last 3 years, but little of the new wealth has been distributed amongst the majority of the population, which is extremely poor.
I favor both sets based upon the probable growth in demand for stamps of Belgium and Colonies. Given the economic situation in the D.R.C., I do not feel that a significant stamp market will develop there for at least a decade. If and when the situation in that country substantially improves, however, the increases in value for its better stamps could be quite dramatic.
Those interested in learning more about investing in stamps are encouraged to read the Philatelic Investment Guide ($5), available on Kindle, and accessible from any computer.