The IPS panels of LG (and Panasonic) LED TVs offer better angle of view but sacrifice on contrast and black levels. In this respect the Sony's and Samsung's are better. But angle of view importance is hyperventilated. IPS panels claim 178 view which is 89 from perpendicular. Will anyone watch TV at more than 50 from perpendicular? Highly unlikely. And even if they do, there will be no joy, regardless on whether picture is impaired or not.
When it comes to 3D, I personally prefer the passive 3D. But 3D viewing will be less than 10%. And at more than 40 angle (from central perpendicular), 3D effect is greatly reduced if not eliminated. But all said, if 3D is very important, stick to LG's passive 3D. Between the two models you mentioned, LM6400 will definitely be better.
Finally, where is the 40,000 difference coming from? Which plasma models are you comparing? From the price front it looks like LG plasma and LG 42LM6400. Without trying to get into the plasma vs. LED debate (oh no! not again!!), I would say it is better to look at Panasonic 42XT50 which is definitely better VFM than 42LM6400 (or LG plasma).