There are a few things to consider when purchasing a bench grinder. The most important factor is the type of wheel that you need for the material you will be grinding. If you will be working with ferrous metals, you will need a wheel designed for that material. For non-ferrous metals, there are different wheels available as well. Another important factor is the size of the wheel. A larger wheel will remove material faster, but it can also create more heat and cause warping if not used correctly.
In general, a 6" bench grinder is going to be less expensive than an 8" model. However, an 8" grinder will have more power and run cooler than a 6" model. If you do a lot of grinding or if you are working with materials that require precision grinding, an 8" bench grinder is going to be the better choice.