You really need to search the web forums and dig out other peoples opinions ... in particular check out the Performance forum on MINI2
and the similar Performance Mods forum on NAM
Personally I have JCW - but at almost $6K it is a steep gift!
The MTH software is very cheap; most people rate it good value for (little) money (<$100?).
There is an AMD Shark Injector which is very easy to install and remove ($400?).
The respected Randy Webb uses a Unichip which plugs in to the engine computer and alters the signals, rather than reprogramming - can be removed ($800?).
Probably many others too; these are the ones I have heard of. As I said above, search ...