Phoenix, Chimera (or whatever the fuck it's called now). It's pretty much all the same, running the Gecko engine. It's just a matter of what u need it to do. Most people need only a navigator, which i would recommend one of the other projects for.
I still use Mozilla, b/c I still use the mail program crom time to time, and sometiems the IRC Chat client.
As long as your supporting open source, it's ok.
They're plans since the 1.0 release was to create a suite of apps, including a calendar among other things. I don't think it's a bad thing, although if you integrate it as such, it tends to run a bit less inefficiently, but that's the cost of integrated convenience.
It's all a matter of which part of the Mozilla suite you need. Most people use it exclusively as a browser and need to download Phoenix and Chimera for the sake of efficiency.