Choosing "Differences..." on the "Reload?" prompt in v15 is effectively the same thing as choosing "No" when using an external difference viewer.

Version 14:
1) File is modified externally.
2) "[file] timestamp has changed. Reload? Yes / No / Differences..."
3) Click "Differences..."
4) External difference viewer loads
5) Close external difference viewer
6) "[file] timestamp has changed. Reload? Yes / No / Differences..."
7) choose Yes or No

Version 15:
1-5) same as above
6) The prompt does not re-appear, so there is no chance to choose "Yes"

Version: 15.0.0.2050
External Difference Viewer Settings:
Executable: C:\Program Files\Beyond Compare 4\BComp.exe
Parameters: %1 %2