Both can be overwritten by a dealer ... and then need putting back on by either a Revo or Superchips agent.
The Superchips Bluefin unit hold a copy of your actual map from your car along with an uprated map. You use the device to reflash the ECU on the car with whichever map you choose. When standard you have reloaded your own original map. The Bluefin device resets the flash counter to whatever it was before you reflashed ... so it isn't obvious you have been using the device
The unit also clears fault codes that were generated during the remap process as they can indicate a car may have been reflashed / mapped ...
Revo overwrites your original map on the cars ECU with a standard map (their version of your std) and an uprated map. You can use a switching device to move between the Revo map and their standard map. Revo is more adjustable if you add further mods like a TBE etc.
There isn't really much to choose between the two but I think most people on here like Revo. I had Superchips Bluefin on my Edition 30 DSG and loved it. A superb map
I had superchips put it on for me and my car (on their rollers) before was 229bhp and after 299bhp. It makes a HUGE difference .... and I miss it having stupidly sold the car a yea ago :(