521 is basically an alternative to the west system at a low price. It has pumps and offers a fast hardener option.
221 is for someone that wants to mix by hand and save money on the project because it doesn't offer pumps or fast hardener only slow hardener.
Both products work great it's just up to the customer what features they want.
221 is slightly stronger than the 521, but there is not enough difference in strength to base your decision on that.