They are actually VERY different films. Pan F has the shortest straight line section to the curve of any current film I can think of. It has the native tonal range of about transparency films, and an enhanced S-curve. That would be apparent if the published curves were extrapolated more. But even in the posted images you can see how the top of the curve begins to shoulder off sooner, and thus the background high values are more open than the Delta equivalent. I sometimes make use of that property in low contrast situation like at the beach on foggy days, when Pan F can lend a wonderful silvery fade-away to the highlights. But for the same reason, Pan F can become quite disappointing in high contrast scenes if you expect well separated shadow values. I think of it as more of a special application rather than versatile film.
As long as Delta 100 is properly exposed for the shadows (which for me requires a rating of 50 to get shadow separation well off the toe), then the contrast per se (gamma) can be developed quite an amount in a manner Pan F cannot. It's a more flexible film, though not to the same degree as Kodak TMax films, which have a steeper straight line further down into the shadows (in other words, less of a toe). In skilled hands, D100 can excel at upper midtone to highlight reproduction, since the curve tends to be a bit upswept, versus shoulder off quickly like Pan F. Again, it helps to see the big picture rather than just "sanitized" curves like Michael appropriately referred to the tiny curtailed published ones.
Slight spectral sensitivity differences, along with different filter factors, are evident from the spectral sensitivity diagrams. Speed of course is quite different. And Pan F can hold even more detail than Delta. I personally develop both in PMK pyro, which lends a lovely "wire sharpness" rendering to Pan F, and with both films, this particular staining developer reins in the printing quality of the highlights better than ordinary developers.
