I find that the proper choice (one finger or two--and which two) is very context-dependent. Using two fingers produces more awkward, clustered positions, and leaves fewer fingers free for playing passing or grace notes (as well as limiting them to one side) or for preparing for a following chord, run or leap. While it facilitates legato in some cases, it impedes it in others. It also requires that at least one finger be moved out of its natural positioning, and often requires more effort, contrary to the conservation of effort/movement that characterizes the most musical playing. In other words, the two-finger fifth is also a limiting technique, just as limiting as the one-finger fifth though in different ways. Neither approach is inherently superior, and neither should be eschewed.

Consequently, I'd advise learning to play both fingerings at roughly the same time, and seeing how a change in context affects fingering preference. As Dick implies, learning the two-finger fifth should not be deferred, nor should it be considered a special case alternative to the one-finger fifth. But the solution is not to denigrate the one-finger fifth, which very often proves a simpler, and therefore better, choice. A player should finger according to what he IS going to do, not according to what he COULD (but won't) do. This can present a challenge for tutorial materials, since they should follow best fingering principles in all but explicitly preparatory exercises. This means that they may have to introduce variations in beginning materials to produce real conditions that naturally favor two-finger fifths over one-finger fifths.