For someone just starting out (like the OP), this is always the dilemma. Powder or primers -- you need some of each before you can start. I don't advise buying powder in bulk to begin, because you may decide that you're not happy with the performance you're getting with [__fill in name of powder__] and the bullet you know you want to shoot.
You're looking to load for .308.IIRC, that takes large rifle primers. LR primers work with any powder you'll use for that cartridge, but no other primers will work. (Okay, technically large rifle magnum primers may work, but aren't recommended.) I would view it as a one-to-many relationship. The LR primers will work with any powder, so you're safe to buy those in quantity. You may find yourself experimenting with powders, so for one cartridge/caliber, I would not start out buying large quantities of powder.