Mantis and Redmine, which one is better for issue tracking?

I can recommend redmine. I've been using it for more than 2 years, with 25-50 simultaneous users and more than 50 projects.

I went through a lot of updates without ever having any problems.

The database is properly normalized, so if you ever need to retrieve any data, you will be able to do so.

Numerous plugins exists which may cover special needs if there are any.

---

Edit: In the meantime, I had to change over to Jira, but I'd go back to redmine anytime if I could.

Never used Redmine, but we've been using Mantis for about 7-8 years for many projects for our distributed team. One of the benefits is its simplicity. We've even wrote a couple of our own extensions, e.g. widely used in our process Kanban board (one of the Agile approaches).

Sometimes I think it looks slightly outdated among other modern tools but it really works for us and we can extend it with our own PHP code.