the cheap hdmi cable you get with the dvd player etc will give the same performance as a very expensive cable because its a digital signal, if it were analogue a good scart cable would make a difference
don't let a shop push you into buying upmarket ones they make more on these than the dvd player itself in most circumstances
no idea why they keep falling out try to fix them with some cable ties to somewhere so the cables dont move i had to do this with the infamous scart plugs