I tend to buy bosch professional, but it all comes down to how often you use it.
I once bought a 4 1/4 inch angle grinder from Machine Mart for £14.99 or something, it worked fine, a bit toyish but the motor was powerful enough and the ground and cut as much as I asked of it, for a occasional user it was probably ideal, it got stolen or I would probably still have it. It even came with spare brushes.
I then got the bosch ones and have done a bulkhead change, chassis removals, disco rebuild, fencing work, general metal work and don't see an end to it's use yet, deffinatly worth the money but only because I use them a lot.