I have since discovered that Win 748 is slower burning then 760, but again it begs the question: when is a fast burning powder more desirable then a slow burning powder and vice-a-versa?
Well, you've got that backwards. 748 is FASTER than 760.
Burn rate charts are pretty much window dressing. They give SOME idea of relative burn rates, but under pretty well controlled conditions. Then we load it behind different weight projectiles in a bunch of different calibers, then the theory goes out that same window. The burn rate varies with the size of the bore,(caliber) and the size of the brass,(standard .308 or 30-06 diameter or the magnum class based on the 300 H&H).
The phrase faster--slower burning powders relates to those burn rate charts. It means little in actual practice.
Most reloading manuals have the top velocity loads at the top of the list of loads they tested. Close study gives you a sense of what to expect for burn rates of the various powders. If they don't list a favorite powder, or the one you have on hand, it means 2 things; 1. they didn't test that powder, or 2. they tested it and found it was too slow, or inaccurate.
Basically, a fast powder uses less to achieve it's top velocity. But it's top velocity is limited by maximum pressure. Slower powders have a longer, flatter pressure curve, that pushes further down the barrel, with a flatter pressure curve.