I'd not heard of Humax before, so I took a look and yours is (I think) a combined Sat Receiver/HDD that receives FreeSat rather than Sky? I believe the picture quality is supposed to be very good. How many HD channels can it receive?
WD TV is a little different in that it is a Media Player (just like Windows Media Player on a computer than plays video content) that enables you to view films/tv shows that you have downloaded to your pc (legally, of course!) on your tv.
It connects to your tv via the HDMI socket. Also connected to the WD TV box is an external hard disk drive that contains the films/tv shows etc that you want to watch. So, to use it, you switch WD TV on, it looks on the hard drive and displays on your tv screen a list of all the films/tv shows stored on the hard drive, you then click on the one you want and watch it on your tv.
Basically, its the first step towards Internet TV - in a couple of years, the whole idea of watching tv via a satellite dish or freeview aerial will be obsolete - your tv will be hooked up to the Internet connection and all programmes will be streamed through there.