It seems that it is a popular question. I want to share my humble opinion to initiate the discussion.
First, compare the objective technical spec.
ZF25 - cheaper, slightly lighter, focus closer
ZF28 - 1 stop faster, floating element
Then, look at how Zeiss describe these 2 lenses
ZF25 - The powerful all-round wide-angle lens
ZF28 - The high-speed artist
If we look at the MTF diagrams in details, you will find that these 2 lenses have completely different characteristics, especially after stop-down. ZF25 is gradually dropped towards the edge while ZF28 has a big drop and then a kick.
Technically, I think ZF25 is a better lens, especially on a DX format DSLR which cut off the not-so-good edge resolution. But, based on my past experience with CY28/2, the drop and kick of ZF28 will add more 3D feeling if using properly. Place the subject at the center. The surrounding will not have as much details as at the center (look at 40lp/mm line) but the contrast remains the same (look at 10lp/mm). The result is more similar to our bare eyes. Also, ZF28 is a floating design which, in general, has a better resolution and, more important, distortion control at close range. It makes ZF28 a good portrait lens at close range.
To conclude, choosing ZF25 or ZF28 depends on your intended usage. ZF25 is good for landscape (which you want to be able to count the tree branches and leaves). ZF28 is for low-light, close portrait and "小品".