Well chaps (and chapettes), the main mechanical differences are as follows;
the BZR has a 6-speed box, LSD and superstrut suspension, which I believe the ST205 Celica GT4 also has... whereas the BZG has a 5-speeder (which is supposedly stronger), pikey open diff and macpherson struts all round. And they both have the exact same engine.
I think the superstruts are just more track orientated suspension straight from the factory. Not sure why they apparently go wrong all the time though. I've been looking into these for a little while now for the lady-she's got her G-wagon up for sale (yes, we have 2 silver ones between us). There's one for sale round my way that we quite like the look of, it's a black BZG and should be round about the same as the EK4 we used to have.
Kev