As Caoimhe said, if well used they are both helpers, they help to reinforce your aids! There are a lot of spurs to all types of horses and riders! I have different spurs for diferent horses, I use them everyday!
I don't think spurs and crops should be compared. Used properly, spurs should refine the aids, whereas a crop is to reinforce the aids. With proper use of both, I am not against either. However, I do believe they should be removed from people who cannot use them.