Basically subtle. They claim a slight increase in gain (sensitivity) with the IDX Pro, and having owned both of these models (several of each), I can tell you that most will never see the difference in performance.
The most obvious difference that will be seen is the decals are different, and especially different is the decal under the display. The IDX has the silvery, shiny decal that creates sun glare, while the IDX Pro's subdued, non-glare black decal doesn't create that problem.
Now, if there is an advantage to the 'original' IDX, it is this. If a person opts to have the modifications done by Bill Crabtree at Surfscanner Detectors, the resulting sensitivity boost will result in a unti that is just a titch 'hotter' than the factory IDX Pro.
One very basic fact remains ... regardless if it is an IDX or an IDX Pro, it is one of the more versatile, great-performing detectors you could own!
Monte