Both are really easy to understand even for a foreigner as myself. Both readers are comedian with a very good pronunciation.
I prefer the Jim Dale version because of the various voices he is performing.
Hagrid is a very convincing half-giant and I'm loving McGonagal scottish's accent. And the sorting-hat's songs...
I found as well to my great surprise that the text read by J. Dale is much more extensive than the one read by S. Fry. It even includes some text not present in the French translation for instance.
Steven Fry is better at using inflection in order to outline part of the text. This is sometime very pleasant and really missing from Dale's version.
The last point is that J Dale is reading faster than S Fry. To me it looks like a "plus" as I am finding audiobook way to slow. (Especially as compared to my reading speed which is easily 4 time faster.)
Hope it will help you find the version you will like the most.