Buffs - Potency: Most songs scale in potency determined by the total amount of skill you have. Compared to 1 String or 1 Wind, 1 Singing Skill will add the same amount of skill in relation to buff potency. In otherwords, If you need 5 more skill to get to the next march tier, it can be overcome with 5 Wind or 5 Singing, or even 2 Wind and 3 Singing.
Buffs - Range: The range of your songs when using a Stringed instrument is solely determined by your string skill. No amount of Singing Skill will increase the ranges of your songs when sung by a string instrument.
Debuffs - Potency: Instruments determine potency. More research needs to be done on Requiem. Wiki says it increases in damage, but attributes it to an increase in level, not skill. It is possible that a certain amount of skill may increase the DoT effect. But meh, its requiem.
Debuffs - Accuracy: Tests have been done, but nothing conclusive. For the most part, you'll want a nice ballance between the two. This is my opinion given the fact that, like most things in the game, too much of one thing leads to diminishing returns. I've personally seen tests proving instrument skills are better and some proving singing skill is better. Best option is to try to find a nice ballance for yourself without sacrificing too much.
Your question was somewhat vauge. If this didn't cover it, pls clarify.
Asmoranomar - Diabolos BRD/RDM
- Song Cast Time Setup BRD/WHM
- Wind Buff Setup BRD/WHM
- Wind Debuff Setup BRD/WHM
- String Buff Setup BRD/WHM
- String Debuff Setup