Well it's basically like this. We use mdf as much as possible in the machine and HDPE for anything that comes in contact with much of the glides and lead screw assemblies.
The router holder, motor mounts, gantry adjustment box in the carriage and on the gantry side are hdpe, and all of the bearing and acme nut holders or blocks whatever you want to call them are hdpe as well. This machine was designed for MDF but I'm sure it could be somehow modified as an aluminum machine. In fact I wouldn't mind seeing it for myself . It is more of a hybrid machine than anything. There is an even distribution of both metal and plastic and mdf in the machine making it rock hard, solid and durable.
But if we're talking about weight, yeah if you dump all that MDF for aluminum the only thing you're going to gain is price, but you will lose a significant amount of weight. I just like the idea that it's mdf because I'm a woodworker, and I can modify it and work with it, and most of all build it with woodworking tools.