I haven't noticed any difference between these two flours in sourdough. Anyone else found this?
While using those flours in a starter you might not notice much of a difference, unlike for bread baking, a difference on how the loaves may take to water would be apparent.
The bread flour has a higher protein level and thus would absorb more water.
While feeding your starter I would just stick to the basic AP flour since it's a bit easier to come by and the bread flour could serve a better use such as for bagels or a nice multigrain bread needing the extra support that bread flour could give it.
When I first started baking sourdough, about a year-and-a-half ago, I found loaves made with predominantly KA bread flour were very chewy. I like it that way, but my wife objected. Since then I've used a 50-50 mixture of KA Bread Flour and KA AP flour in sourdough's along with 10% whole rye flour. Their crumb is less chewy, and my wife stopped complaining.
I know that bread flour is stiffer and takes more water, but I could discern no difference in baked loaves when using straight all purpose flour vs. a loaf with 28% bread flour. I subsequently did a test mixing the same amounts of each flour and water and kneading. They were equally stiff and responded equally to a measured pressure applied to each. That is why I wonder if they are (virtually) the same.
Make loaves out of each pure flour and you should easily sense a difference which will be evident in both the dough (assuming same hydration/recipe) and the bread (chewier, most likely tighter crumb for the BF). The difference in protein content of AP and AP with 28% BF is only about a half a percent. Enough to be noticeable if you have a good deal of experience with say the pure AP version but rather subtle and not distinct enough to go "Oh WOW!" Even pure BF and AP are not so distinct that you will necessarily feel a big difference.
I haven't used it for obvious reasons...I live in the UK!
However, King Arthur list full details of their flour analysis on their website. I don't say you would not notice a difference between these 2 flours when used in exclusivity to make bread. What I would say is that the AP actually has all the characteristics of a perfectly good bread flour in the first place. If it were made available in the UK, for instance, it would be far closer to our Strong flours, maybe superior to many! than it would to our "Plain" flour. My point really is that I view this particular AP flour as a bread flour. I do repeat, that this is based on the KA analysis on their website, not personal experience.
I used to bake exclusively with KA BF, then tried the AP and noticed a difference in the crumb (I bake only sourdughs).
KA BF is milled from hard red spring wheat and has a protein level of 12.7%. KA AP is milled from hard red winter wheat and has a protein level of around 11.7%.
I've found (as have family and friends), that the AP flour results in a slightly softer crumb. So that's what I primarily use now, although I do sometimes blend BF and AP, especially when using a higher hydration formula.