Both poets highlight nature’s spirit and they both end in a hopeful note** though they do this in significantly different ways, as these notes suggest. You could define/describe these differences, variations on the way “Nature” provides inspiration & hope for the speakers of both Coleridge’s and Shelley’s poems. IF you can decide what you think’s important or significant about the differences you’ve identified & described, you’ll have the foundation for a specific thesis you can argue for: the pattern is something like ’though similar in these ways, different in these other ways, and that is important because…’ That’s not a model for your thesis sentence, it’s the thought process that gets you there!