I have a Caddx NX8e that I installed myself with a relay output expander board. I also have an expander input board for a total of 24 zones. I have motion sensors, glass break sensors, and your typical magnetic contacts for doors and windows along with three keypads attached to it.
One advantage I have is the Caddx NX8e is interfaced with my HomeSeer system via a plugin and I can perform arming and disarming tasks via HomeSeer as well as monitor all of the 24 zones (and perform actions/events via HomeSeer on those zones).
I also purchased an Elk M1 Gold for my buddy's house and will be installing it in a few months. I've been playing around with it on my bench, but don't have a lot of time involved with it yet. I've also played around with the ELK software.
I would say that your comment on "magnitude" better would depend on what exactly you plan on doing with your system as far as home automation tasks are concerned.
For instance if you just planned on using a system for basic security then either system will work well.
But, if you want to integrate voice announcements, analog measurements, W800 RF interface, logical "rules" based on various inputs, etc... then the Elk is the way to go.
How extensive is your security system now (glass breaks, motion detectors, zones, etc...)? Do you think you would ever want to integrate an 'action' that would be triggered by a motion sensor or window/door opening?
The Caddx can do logic integration and does have a relay output board, but the programming interface is not at all user friendly. I actually have my Caddx relay output connected to my Ocelot's SECU16 digital input module (the Ocelot is much easier to program and a lot more versatile). I also have the Ocelot's relay output tied to a zone input for the Caddx.
If the Elk M1 was available years ago, then I wouldn't have had to integrate these two systems (each with its own programming language) together to get the home automation flexiblity I wanted. I'm a big believer in NOT having critical tasks delegated to a PC!
If money is an issue, then stay with the Caddx and beg Dean to develop an interface for it. There are still a lot of Caddx users out there, but I'm betting every one of them wishes they had an Elk M1.