The most obvious difference is the tuning slide, which for those that are performing is often a necessity. The tone holes on the Kelpie are also optimised for ergonomics, whereas the Pros are optimised for tuning. Both are, however, great instruments and if you look at the price of other musical instruments you get a lot for your money. For this reason people often just decide to treat themselves to a Pro, though there is also a lot to be said for starting on a Kelpie to see how you get on, and then if are using it a lot, then you can treat yourself to a Pro or another Key knowing that it's going to get some good use.