100Hz has nothing to do with input lag, which is what, you refer to.
Decide, which is more important: Gaming or regular TV. There are many parameters affecting the choice, input lag is just one and it only affects gaming (but not in a small way).
A plasma has no input lag, but currently has, in my opinion, inferior postprocessing chips for regular TV or movies. An LCD (or LED-backlit LCD, if you prefer) has to add some processing, and you would have to look for one with a 'game mode' to minimize it. Plasmas are generally considered to have the best picture quality, however, but can have other drawbacks.
You could hardly get a 50Hz LCD nowadays, if you tried, but it is far from certain that above 100Hz would yield much visible benefit. 3D.. if you want to go there, there's even more to consider.
40" from 3 meter is fine. Any further away and highdefinition (bluray) will seem like regular definition (DVD).
Contrast ratios can largely be ignored, as the black-to-white contrast numbers are not very useful (because it is the undisclosed grey-to-grey contrast along with rise-fall time, that matters) and are often not even comparable across manufacturers. Then there's the local dimming vs. edgelit vs. plasma consideration - and design, price and other things, as well.
Good luck compromising ;) or you could also avoid the "analysis cost"/hassle and just to to Conforama or Toppreise.ch and pick one.