None of the 2 model numbers you mention can be found on the US site: http://www.lg.com/us/search.lg
Do you live in another country? Did you try your local LG site?
If these are products you can buy where you live, any shop should be able to tell you the differences. If they can't I'd go and find a better shop.
I can't find any review about the LG LM640 models, and also I can't find the differences between the LM640t and the 640s. Can anybody help me?