setTag vs setExtra

I was wondering, between these two, what would be better for storing submission IDs (think a single form submission)? For one of our projects, there are tens of thousands of submission IDs per month. We often use these to identify issues (so a client would provide us with the submission ID where they saw the issues). We would need to use this to search. Is setTag or setExtra more appropriate for this scenario? What are the pros and cons of each?

setExtra has more liberal size limits, while tags are searchable. I think you want tags.