I use a car navigation system, (a Mio Moov, was cheap, like $100 if I recall) and it runs LK8000 great. The display can be a little hard to read in the sun but the new airspace views of LK8000 (both top and side) have been fantastic... our gliderport is underneath the edge of a class B so it's been really helpful to keep track of where I am regarding the airspace above.

Also, clicking through the screens is fast and easy.. the earlier versions of XCSoar (I can't speak for the newest) required clicking tiny little buttons and scroll bars, and thats something that just isn't going to work for me in the cockpit. LK8000 is designed around quick and easy navigation between screens with minimal distraction.. in fact you can identify screens by the sound they make as you switch without looking.

Try both is my advice... read the manuals for both... and watch youtube for examples of how things are done, then test fly them in sim mode or on a simulator.