Submarine has a slightly different approach to Highrise - it focuses on minimising data entry, so emails with a contact are automatically stored without having to remember to bcc the app. Similarly, it also auto-archives conversations you have with contacts over Twitter, which I don't think Highrise does.
It's got quite a big focus on search too, so you can run reasonably complex searches like "journalist twitter followers > 2000 covered:yes last emailed more than (1 year ago)", which will show you (as you might expect) all the journalists that have more than 2000 twitter followers, who've previously written about you but that you haven't spoken to over email in more than a year. There are other commands too, and I'm planning on adding more over time.
Finally, I think the Chrome extension is pretty cool - the image on the homepage does a better job of showing it than I can explain, but it gives you a little button in Chrome that, when clicked, pops up with a contact's details if you're on their website (or Twitter, Facebook, Linkedin etc profile).
