If you want to be able to monitor it inside the house, in real time, using a computer, the Mate 2 is the only option at present. The Mate 3 will eventually have firmware that allows you to access its data over a network connection, but that isn't available right now.
If you'd be content standing at the Mate 3 and looking at graphs on its display, the Mate 3 would be an option, but the data resolution is not as fine, or else you start to run out of memory for the data files.
The Mate 3 is more of a traditional data logger in that what it does is log data. Something else has to post-process or analyze that data, provide reports, etc. This is what the greenMonitor (what my company makes) and WattPlot (from Andrew at Intallact) software products do.
Our software can run on a Windows or Linux PC (we have little demand for Mac OS X and that support tends to lag behind as a result), or a low-power data logger (greenMonitor Lite) with more advanced capabilities than a Mate 3. WattPlot must be run on Microsoft Windows, either on a Windows PC (native) or a virtual machine running under Linux (VMware or Xen) or Mac OS X (Parallels). Intallact doesn't have an embedded hardware solution, but for people who like the WattPlot display, but don't like the power consumption of running a real computer 24/7, the greenMonitor Lite can be used with WattPlot. That set-up is pretty straight-forward -- use WattPlot to look at the system when your computer is up and running, and let the greenMonitor Lite keep track of the data when your computer is turned off or somewhere else.
If you shared a bit about what you want to do with the data, that would be helpful.