HI Bill,
Both of these concepts are correct. First, human senses cannot detect the difference between grain sizes. Our eyes and ears are just not that good.
However that does not mean there is no difference. In tests where I ran 20 trials with 6 different powders, the slowest Swiss Null B time recorded was faster than the fastest Goex 3fg time recorded.
I do prime from a flat horn carried in my pocket. I like fast, but the main reason I do this is because the flat horn is handier for me than handling the main horn and stopper twice. Even if I carried 3fg in the priming horn, it would be easier than handling the main horn twice.
Regards,
Pletch