With Carista you pay a license fee for the app but be aware that it's a recurring annual fee of $40 so it's not just a one-off payment. The Carista app works with most OBD2 compatible Bluetooth or WiFi dongles, however they do sell their own Carista branded dongle which is guaranteed to work without any issues (available from Amazon UK
HERE at a cost of £32). Carista works with both Android and iOS devices.
With OBDeleven the app is free but you need to purchase a specific OBDeleven dongle for your car which is either 36 Euros for the standard version (fault code scanning only) or 55 Euros for the pro version (fault code scanning, coding/adaptation, and output tests). This is a one-off cost so there is no recurring annual fee to pay. In addition to this, if you want to use any of the internal "apps" (these are used to perform one-click operations for some coding and adaptation settings) then you need to buy in-app credits. You don't have to use the "apps" if you don't want to and it is still possible to manually perform any of the coding for free that the "apps" do in a one-click operation, but you will need to figure out yourself what coding you need to do (and how). OBDeleven only works with Android devices. The company is based in Lithuania and you have to buy the dongle direct from them. There are no UK suppliers although the shipping from Lithuania is free.