This isn’t surprising given that save for this year’s flagships, previous Samsung phones have been powered by both chipsets, although they were separated by region and market. However according to the latest rumors, it has been suggested that for the Galaxy S7 models in the US, we can look forward to both versions.
This is based on an alleged list of model numbers which reveals both the Galaxy S7 and the Galaxy S7 Edge as being SM-G930 and SM-G935. The list also revealed the model numbers for Verizon (SM-G930V, SM-G935V) and Sprint (SM-G930P, SM-G935P) as being the Snapdragon 820-powered models, and AT&T (SM-G930A, SM-G935A) and T-Mobile (SM-G930T, SM-G935T) as having the Exynos 8890 chipsets.
The AT&T model numbers corroborate an earlier claim by @evleaks who suggested that both variants are apparently undergoing testing by the carrier at the moment. No doubt the difference in choice of chipsets will cause some debate and it is unclear as to why Samsung would want to do that, but either way it is a rumor so take it with a grain of salt for now.