Both available from Mozilla, is one better than the other (ie performance, security)?

SeaMonkey isn't actually maintained by Mozilla, but by The SeaMonkey Project, an independent community of developers; it's a continuation of the Mozilla Suite, which was dropped by Mozilla. It's different because it's not just a browser; it includes an Internet browser, email & newsgroup client, HTML editor, IRC chat and web development tools. If you need all that stuff, it might be a good choice.

The one downside is, it tends to be slightly behind Firefox in updating. I don't think it's a really big deal.