Difference between Thesis and Research paper?
An undergraduate thesis (or dissertation) is: a document submitted in support of candidature for an academic degree, which presents the author's findings.

By comparison, a research paper (when differentiated from undergraduate life) is: an academic work containing original research results or a review of existing results, which are typically published after a peer-review process successfully checks suitability.

For an undergraduate, I suspect an undergraduate thesis is easier, since I believe the bar for acceptance is generally lower than the bar for acceptance of peer-review.