I think we're really dealing with 2 separate issues here.
1. Given the if infirmities of many of us and infrequency of soldering practice, the ease of soldering and ability to produce a clean joint is of prime importance. Regarless of chemical, metalurgical issues, a cold joint will not sound as good as a good one. That means for me, a eutectic composition (where the melting point is below that of the individual components and the liquid state goes directly to solid). This leads me to solders, Multicore, Kester 44, Cardas, and various other eutectic mixtures, primarily composed of lead/tin, often with additions of copper, silver..... And the type of internal flux can have a major effect on flow and wicking which can make or break your efforts. And if you can source it, there is the ternary tin-copper-silver eutectic from Johnson Manufacturing IA-423 (with organic flux), mp 423 F degrees.
2. For those who can compentenly weild a higher thermal mass iron, non eutectic and non lead bearing solders are feasable. Many prefer an 4-8% silver - tin composition, Kester Sn95 Ag5. But frankly, I lack the dexterity/competence to deal with such solders.
So how would we go about judging the merits of differing solder/flux compositions? If we assume the benefits/negatives of a given composition is cummulative, I guess we could assemble 2 of the same component, and listen for a difference between them with 50+ solder joints of the same solder on each hopefully yeilding a discernable difference.
