Let's,,,,,,
this is more like..do it (the person is sure about something)
and
why don't we...
this is more like.. I want to do this, and what do u think? (kind of asking )]
I HOPE THIS HELPED U^^

Both are similar.

"Let's" is stronger. Although "let's" can be an invitation, it's not used that way as often as "Why don't we".

A child may say to his friend, "Let's play!" Although he is inviting his friend to play with him, "Let's..." is not a question, so it doesn't require an answer.

However, let's say that you want to go to a restaurant with your wife (see how I used "let's" (^_^) ). You might say, "Why don't we go out to eat?" You are inviting your wife to go out to eat, and because you made it a question, it requires a response.

"Let's" is also said by someone that is confident that he will get a positive answer. The child is pretty confident that his friend will want to play.

"Why don't we" is said by someone that is not as confident that he will get a positive response. For example, maybe your wife is sad and you want to take her to a restaurant to cheer her up? If you say, "Why don't we go out to eat?", you are not sure she will say yes. Maybe she is too sad and just wants to stay home?

I hope this helped.

I got cleared. thanx!