[Grammar] Why not do it versus Why don't we do it

[englishhobby]

1) Why not go sightseeing?
2) Why don't we go sightseeing?

or

1) Why not buy this dress?
2) Why don't you buy this dress?

Is there a small difference in meaning between 1) and 2)? Can it be so that the 1) is just a suggestion while the 2) is also a suggestion, but based on a bit of very mild criticism or annoyance (We have been here for three days already and haven't seen a thing! Why don't we go sightseeing? or You have tried five dresses already! Why don't you buy this dress?)

Or are 1) and 2) absolutely identical in meaning?

 

[MikeNewYork]

1) Why not go sightseeing?
2) Why don't we go sightseeing?

or

1) Why not buy this dress?
2) Why don't you buy this dress?

Is there a small difference in meaning between 1) and 2)? Can it be so that the 1) is just a suggestion while the 2) is also a suggestion, but based on a bit of very mild criticism or annoyance (We have been here for three days already and haven't seen a thing! Why don't we go sightseeing? or You have tried five dresses already! Why don't you buy this dress?)

Or are 1) and 2) absolutely identical in meaning?

As you have suggested in your post, context is very important in determining meaning. In spoken English, intonation and stress can also add information. The only difference I see in the first two is that #1 does not seem to include the speaker. The second two mean the same thing to me.