Two versions of cation softener resin are sold on this page: standard and fine mesh. Unless you know that your softener was originally set up for fine mesh resin, buy standard (Hi Cap). Standard Hi Cap is the preferred resin for regular hardness removal.
Fine mesh resin also removes hardness but is preferred for removal of iron from well water. (Standard Hi Cap also removes iron, but fine mesh does it better.) The disadvantage of fine mesh resin is that it reduces service flow. You cannot simply put fine mesh resin into a standard unit. You'll need to change to a more restrictive riser basket to avoid loss of resin.
To summarize: if you have a standard water softener, buy Standard Hi Cap resin.
If you're comparing resin prices, remember that we pay shipping. Resin is heavy (53 lbs. per cubic foot).
(whole house & well units)