It would be hard to find a better push-dagger than the Cold Steel. That said, push-daggers limit your self-defense capability pretty much to punching techniques, and offer relatively little daily utility.
Personally, I would go toward getting training with the defensive knife, and going with something more conventional, such as Cold Steel's excellent Voyager series. You'll have a much more versatile defense tool, AND a cutting instrument you'll find handy to use and have with you on a regular basis.
There's also the element of legality. Knife laws are VERY strange and VERY fractured, both in the US and around the world. There are LOTS of places where it's illegal to carry a "dagger" of any kind, but perfectly legal to carry a folding knife like the Cold Steel Voyager.