bootable backup & two-way sync?

[pdxcarl]

That would be a great solution if it were only data files I want to have available. However, because I also want to have all my software available, and specifically because I don't want to have to go through the headache of installing software, deleting software, updating software, trying to remember to match the settings, preferences, etc, etc on two different systems, perhaps that will provide some insight as to why a "data only" storage solution on the external drive doesn't work for me.

What I want is my computer, configured my way on an easy to carry external hard drive. Software exists to sync the two drives both ways (Synchronize! Pro X, Synk Professional, ChronoSync, etc), but I don't know how to get it to "play nice" with my bootable backup configuration. I suspect that whatever solution I come up with will have to be a hybrid solution, wherein I select only the directories (Applications, Home (User) directory, and whatever else might get changed throughout the day, including preference files, permissions, etc, etc), but I'm not sure what to sync (and what to leave out) and how to go about it in a way that will maintain bootability and not mess anything else up.

The net result should be that whatever is on the external should exist on the internal (and vice versa). This accomplishes my goal of portability and an up to date backup of my system.