It seems that sodas and fruit juice have the same pH level - about 2.5 to 3.5. This question is about the biological difference although I understand that the chemistry (e.g. carbonic vs. ascorbic acid) may enter into the discussion so basically: what is the difference between how acid in sodas and acid in juices affect / are affected by your stomach?
Juices primarily have citric acid and malic acid. Soda may have these too but cola has phosphoric acid.
All these drinks are less acidic than your stomach acid and will dilute it.