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WHen I need to choose a new powder I go straight to Lymans manual and look at what powders they used for their data, for the round and weight in question. They use the powders that get published that give them the best results. They list which powder gave the best accuracy in bold type. This is a fairly bulletproof way to choose powder even you do not know much about burn rates. (still a good idea to know though).
There will always be loaders who choose obscure powders and report fantastic results perhaps, but they are usually advanced re-loaders trying to make something very specific for their needs. For newer reloaders with less experience, or for general purpose range ammo and such, going by the book powders is the preferred method.
Barrel length sorta plays a part in it also. Lymans loads are listed in order of burn rate, with the faster powders at the top and descending. Loosely speaking, longer barrels will like slower powders and short barrels will do better with faster powders. Unique will work good in your 9mm.
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