You may think that by asking a limited question, "Are these sentences equivalent", you are avoiding most of the problems that come with natural language processing.

That simple question has only black and white answers, and that requires 100% certainty. And that implies a very great level of understanding. For example:

• This is the thing.
• This isn't the thing.
• The thing is this.
• Isn't this the thing.

Four sentences of just four words each and only two characters separating them, but four completely distinct meanings. And you could substitute any noun for 'thing', and the sentences remain equally distinct.

I do not think that there is any statistical measure or weighting algorithm that will allow you determine their equivalence or lack thereof, nor even give a probability of equivalence that would be useful in determining that.