I guess a good starting point would be the Miles&More forum. There you can read about the service and comfort available on LH/LX/OS flights. I have had great experiences with LH on both paid and award tickets where I have been given op-ups(in my defense, I fly enough to keep *G on US and *S on LH). You just have to find out for yourself. To Prague, your best choices would be LX or LH and LH is more constant.