Most of the Questions or Answers (Q/A) on the review queue have something to improve (title, tags, poor content formatting, etc.), but sometimes are Q/A that are, not just well written and I don't see anything to improve there, but also are useful so I want to upvote it.
So the main question is, What should I do in such a case?
Don't do this.
Upvoting is a legitimate review, why would you skip it then? Skipping it does not count it as reviewed by you, and just keeps it in the pool.
Skippping should mostly be used when you're not sure.
For example: You remember something similar being around, so it might be a dupe, but you can't find it / pinpoint it -> Skip it. It feels like this should be closed, or dealt with, but you can't really nail down how or why.
If you believe that the upvote is all that's needed, then do this.
I'm Done is really you marking it as reviewed, so, if the Upvote is a sufficient review in your eyes, go with it!
This is mostly for posts that are "fine", just neither upvote-worthy, nor in need of improvement. If you feel uncomfortable with upvoting it, because it feels not "perfect" in the sense of "This deserves an upvote right now!", use this.
Skip, because it will continue going around the queue.