The industry trend in my area of interest (upstream oil and gas) is to specify the Aflas in moderately sour service and / or in amine service. Buna does not last as long or perform as well in those services, apparently.
There is a world of difference in the materials, Aflas being a Fluoropolymer and Buna-N being a Nitrile Rubber. Though both have their limitations the Aflas covers more ground, though at a higher cost.
A key point is if you are able, is to check it out in the fluid you are going to exposed to. A simple immersion test will tell you a lot as most charts are for pure materials.