Both can make a big difference.
With an '06 sized case, brass can make a 2 grain difference it what's safe and what's not.
I've seen primers make 100 fps difference. I've also seen time where it makes no statistical difference.
So, it depends. Until you know what your doing, it's best to start low, work up, and build your data set.
Good post AS. I've also seen a huge amount of difference in capacity, when comparing different manufactures 30-06 brass. One in particular, that I have and should get rid of, is some old UMC remington brass that is 3 grains less than even newer Remington brass. As for other manufactures: Winchester is known for being thin as well and FC known for being thick. Case volume varies greatly, especially when working with cartridges that were designed for military use. As far as primers, I started out using CCI and I have not swayed from them. They just work very reliably and I get good dependable accurate loads... When I load, I always refer to the bullet specific manufactures load manual or online data. That's always been a safe route and one can't emphasize enough, the need to start low and work up.
