im not saying that the decision to construct the planes out of titanium wasnt valid - im saying that some of the info regarding aluminum was wrong - it may have come down to an issue of tensile strength - titanium is certainly higher, perhaps it was fatigue that was considered. Aluminum does fatique and crack- this is the reason titanium engine valves exist but no such thing as aluminum engine valves (well in addition to heat when you consider the exhaust valves).
Also consider that aluminum has to be used in greater quantities and with a more bulky structure to acheive the same strength as say steel or i imagine titanium alloys (im pretty sure pure titanium is quite brittle) If you are away of motorcycle frames and the comparison of an aluminum twin spar delta box frame and say a ducati trellis frame you know what i mean. Aluminum fatique is one of the reasons aluminum is just now appearing in dirt bike frames - the shocks and jolts cause cracks in aluminum but steel can deal with the vibrations just fine (newer technology has now enabled aluminum to be used in dirt bike frames)