Ignoring the which-is-better thread, I fitted an electric fan from famous four about 3 years ago.
The fan is made by Spal and looks the biz, their mounting kit is pretty good, though nothing you couldn't do yourself, and the thermostat is of a similar design to Si's, as far as I can see. Open to corection as I haven't seen the X-Eng one in the flesh.
The thermostat did die a month or 2 after I got it, but was replaced FOC.
If I was doing it again, I reckon I'd go the scrappy route, controlled by X-Eng.
oh, and Electric is better