The most basic decision that most collectors make is whether to collect stamps mint or used. In the very earliest stages of collecting either will do and, at the very highest stages, finances often force either to do. But most collectors start saving both and then gravitate to one or the other. The decision comes down to a calculation. Used stamps are usually cheaper and require less care in mounting and handling. If I were putting away stamps long term for an investment portfolio I would only select used as with mint stamps gum grading standards have changed dramatically over the years and are likely to do so further in the future. Gum can degrade over time if not stored carefully and even the smallest amount of humidity can cause toning. Since so much of the value of a mint stamp today is in  perfect gum I wouldn't want to have to worry about this as a long term investment. Used stamps are easy to store and harder to damage. You probably shouldn't try this at home but if you ever have intentionally tried to crease a stamp, it is far harder to create a crease that shows up in fluid on a used stamp than a mint one. That is because paper fibers are a bit forgiving and a stamp without gum and which has been soaked has a certain "bounce" in the paper that makes creasing more difficult. Too, used stamps are usually less expensive than mint and a collector or investor can get more stamps for their money. Further, ratios between mint stamps and used stamps have changed over the years. Fifty years ago most better used stamps sold at higher percentages of their mint price than they do today. This trend has leveled off in the last decade or so and could well reverse.