They have a lot in common, having started from a common base a few years ago. Both offer much more information and functionality than you are likely to want to take advantage of. Both will provide a superb glide computer system running on the Oudie 2 lite and linked to the Cambridge. My impression is that LK8000 is even more feature rich than XCSoar, since the LK team went on developing new functionality while the XCS team were porting it to Android.

Ideally you would find people at your club using both and ask them to give you a demo. The look is pretty different and so are the gestures/touches used to operate them. If one is more popular than the other where you fly that would a good reason to choose it. Other users can be very helpful.

Whichever you choose, spend as much time as possible studying the manual and playing with the program at home, and try to set it up so you hardly need to touch it in the air. And always fly with it in such a way that if GPS gets jammed or otherwise fails you won't have a big problem.