Can someone tell me some similarities and differences between "Letter to John Adams" and "Declaration of the Seneca Falls Women's Rights...

I think that you are accurate in that the letters between
both John and Abigail Adams and the Seneca Falls declarations highlight the need for
equality between the genders.  Yet, I would point out that there are some significant
differences between the two.  In my mind, the first is the purpose of writing in each. 
The letters between Abigail and John Adams are for private correspondence, and while
they might reveal political discourse, their immediate purpose was not politically
driven.  The Declaration of Sentiments at Seneca Falls was outwardly political, meaning
to make private sentiments purely public.  The letters between the Adams' might have
been limited in this regard because of their nature as being communications between
spouses, relegating them to the realm of the private with public applications.  This is
not even close to the case with the Seneca Falls Declaration of Sentiments, which was
meant to be public, politicizing issues that might have been felt in private or in
emotional realms.  In this light, the purpose, motivation, and audience of both
documents are vastly different.