Dont be fooled by price.
I work with grinding wheels every day and theres wheels that work well and theres wheels that last long.
What you want in a grinding wheel is a combination of both qualitys erring on the side of duribility.
Heres the issue.
Wheels tend to "Pack up" with metal as you use them, this is why they need to be dressed occasionally providing a new, clean surface to grind with.
The harder the wheel is, the longer it will last. Also The harder it is the faster it will "Pack up" due to the fact that it is not breaking down as a softer wheel will. A 'Packed" wheel causes heat due to the material its packed with (metal) rubbing on the material your trying to cut,(metal).
This means you need to dress it to prevent it from burning your cutters.
But you might think, why not just buy the softer wheel and let it break down and not have to worry about dressing it? (afterall, your loosing wheel for nothing by dressing it, right?)
Well, OK, this makes sense until you have an understanding of what happens when a wheel bearks down. (Wait here, I need a beer for this.)
OK....
When the wheel is freshly dressed its fairly "True", meaning round and with a proper radius/angle/square face. And it will stay this way for a while, but eventuially it will become distorted somewhat. this can be caused by bumping the work a lil too hard or a tooth shifting upon contact or whatever.
When this happens the wheel starts to look like a slightly flat tire, ie its not round anymore. This out of round condition causes the wheel to loose contact with the work on every rotation, remember how this whole thing got started?
The wheel bumped the work.
Well the wheel is now bumping into the work with every revolution, and every time it bumps the work it gets a lil more damaged. This process creaps its way around the wheel until it gets to where it started and simply continues to chip away at the wheel in tiny increments.
This happens much faster with a soft wheel than it does with a hard wheel.
Youll actually loose 2-3 times as much wheel through it breaking down on the work than if you dressed it for every chain you sharpened.
Better wheels are just better.