I would say picking the "right" or "best" powder out of a loading manual just comes with experience. Which is not to say that a novice reloader can't make a good selection after inquiring of others, but some times your "right" or "best" powder might not be the same as someone elses.
Having said that, some general thoughts on how I personally pick a powder for a new or different load, after perusing several reloading manuals.
If I have a powder already on hand that seems like it will work, based on the books, that's my first choice. I already have too many different types of powder in inventory and I'm hesitant to add to the herd.
If I have to buy a new powder, or if I happen to have more than one in stock that will work, I tend towards the powder that will give the best velocity/pressure relationship. However, pressure data is not always included in every manual, so you may have to use velocity alone to make your decision.
When choosing from more than one "in-stock" or new powders that are close in velocity/pressure relationship, I look for the powder that gives optimum loading density, IE 85%+ density at starting load and 95%+ at maximum load.
If I'm still unable to narrow it down after those considerations, then I look at, in no certain order;
Meterabilty
Availabilty
Price
Again, after you've gained experience, you'll kind of just "know" which powder will probably work the best for a given situation. This is just my experience and the criteria I use, yours may be different.
W101