My local countrywide farmers shop (chipping norton) has the Blue Bosch Professional Grinder (biggun... 230mm??) for £40 at the moment, dropped from £100 apparently...
It looks as if they do several with different power ratings in each disc size. I'm not sure there's a difference between some of the sizes, apart from the guard. The parts diagrams might clear it up.
What I've noticed is that when you take them to bits, Makita and Bosch have gone the extra mile, ball bearings in places where the cheaper ones use a bush, they've obviously gone to trouble to varnish and protect the windings.
If you are paying a lot for an grinder you are going to use a lot, the weight and the feel are important.
I've found that all Makita power tools are generally well made and feel right.
As for durability, I think a lot depends on the way you use them. If you are pressing down hard on the disc, it puts a lot of stress on the tool and it heats up quicker.
