Actually, the dirt you use depends on the herb.
Some, such as lavender and rosemary prefer well-drained, almost sandy soil. Both of these plants, incidentally, really do well in big pots.
I don't actually buy potting soil/mix very often - usually I get things like (composted) horse manure and mushroom compost, mix that in with kitchen compost/leaf mold and beg for some worm castings and mix all that together with some dirt (er, clay from my yard) and throw in a little bone and bloodmeal and call that "dirt" for my plants.
Anyway - to better answer your question, I'd pick whichever is cheaper, then add other yummy things to it that plants find delicious.
I may be insulting your intelligence here but: Be sure to get pots big enough - I and many other gardeners, as total novices, fell into the idea that we could keep herbs on a sunny windowsill over our kitchen sinks - because we see that in all the home magazines and doesn't that look pretty? We laugh about it now, and I laugh every time I see those pages, suckering more novice gardeners into that cute idea.
It may work for chives and thyme, maybe oregano... for a while... but sage, rosemary, lavender, basil and the like can be 3' tall. There is a popular little Italian restaurant in this area that uses rosemary as foundation shrubs - they're easily each 6' tall and wide