I prefer using the 3M line of abrasives, specifically Cubitron™ II and Cubitron™ 3. The new Cubitron™ 3 line has 3X the service life of the older Cubitron™ II product line and the pricing is the same. 3M has been making abrasives since 1905 so they know what they're doing and I trust 3M more in their ability to prevent catastrophic failures of a disc rotating at 13,000 RPM. That happened once...I learned my lesson.
That 3M sample pack that Ron referred to is a great introduction to the many, many uses of a RA grinder. You get 16 items in some clever packaging as the box has 3 pull-out drawers to keep all the contents in. I put all of the stuff into an extra Systainer I had but there's nothing wrong with the cardboard packaging it comes in.
I also tend to purchase 5" discs over the 4-1/2" variety if I can. The larger discs give a lot longer life and the cost of the discs are usually the same.
For the aluminum cutting, make sure to get a disc specifically made for aluminum as it will not load as quickly as a standard disc. 3M, Pferd, Norton & Sait all make good aluminum cutting wheels.
If you need to cut/grind stainless, make sure that you use the proper
fresh disc and keep that disc/wheel for stainless work
only. If you use a disc/wheel that was previously used on steel, there are extremely fine steel grindings that contaminate the wheel and will then contaminate the stainless and cause rust to form. The only way to remove the carbon contamination is through chemical methods...think acid.
Finally, all discs are not created equal, here's an example. Here are 2 CGW flap discs and a Pferd flap disc. From the top, everything looks the same. From the bottom however, note the additional leaves of abrasive material in the Pferd disc. And from the side, you don't even need to count the abrasive leaves to see the difference. More abrasives...longer life and smoother finish.