See Artscroll Talmud Kiddushin, in the notes they show a map of Babylon, including Pumbdesia and Sura.
The map shows the River Tigris (west of modern Iraq) and the River Euphrates (east of Modern Iraq) diverging from each other like horns.
On the River Tigris, Pumbdesia is lower down (more south to Eretz Yisroel), Sura is is a bit higher up along the river (more north).
Pumbdesia was alot larger both in size and population, Sura was smaller.
Both are mentioned throughout Shas many times (if anyone can jog my memory to where there mentioned-it would be much appreciated).
As far as I can remember Benjamin of Tudela (the famous traveler) in 1160 went at least as far as Bagdad mentions either Sura or Pumbdesia as still existing in his time.