What's the different between "Many Thanks" and "Thank a lot"

i used to use Thanks a lot in my email, but i'm curious is there's any different between Many Thanks and Thank a lot.

i found both definitions as below:

Thanks a lot :A sarcastic thanks, when some one does something that is not helpful for you but rather unhelpful.

e.g Thanks a lot! Now I'll have to do it all again.

Many Thanks : An expression of gratitude, expressing greater gratitude than /wiki/thank_you or /wiki/thanks.

e.g Many Thanks Chris for helping to resolve this issue

from the above explanation, i changed to Many thanks in my email although someone does something that is not really help me.

Please advice.