Probably the main concern when purchasing casters is making sure you get the right kind for the type of surface the chair is being used on. If it's being used on hard floors, you need soft casters and if it's being used on carpeting or chair mats you need hard casters. The other concern is getting the correct stem length and diameter. Match your caster stem to the diagram below for the proper size and length.
After you've figured out those two things, there are other options that you can choose. I know - you're probably wondering why something like this can't be simple, but it really is. The two points above are the requirements - what I'm about to discuss are other options.
Option 1 - Noiseless casters. A dual ring technology eliminates excessive noise. Particularly useful in an office environment.
Option 2 - Safety casters. These casters have internal brakes that prevent the wheels from rolling freely when the user stands up, but allow full rolling when the user is seated.
Option 3 - Instant swivel wheel casters. These casters allow smooth rolling in any direction. They hold up to 250 lbs per set.
Option 4 - Dual usage casters. I saved the best for last - these will eliminate the question of hard or soft casters entirely because they are designed for use on both types of surfaces. They can be used on hard floor, carpeting and chairmats.
That's all there is to it. Now that wasn't so bad, was it?