I am not a kilt maker, so surely you'll receive better answers than this, but in a nutshell:
Pleating to the sett attempts to create the same pattern on the pleats as you would see on the apron.
Pleating to the stripe picks one stripe to be the center of each pleat and can produce some striking differences, depending on which stripe is chosen.

From a thread by Barb T.:

Quote Originally Posted by Barb T. View Post
Every once in awhile, I have the opportunity to post pictures of kilts in the same tartan but pleated differently. It's been awhile since I've done it, and I thought I'd do a quick post for anyone new.

When a kilt is pleated to the sett, it looks pretty much the same fore and aft. When it's pleated to the stripe, however, the back can look very different from the front, and that's part of the charm of pleating to the stripe. And different choices can have very different appearances. If you intend to pleat a kilt to the stripe, it's worth pinning up several alternatives and then standing back a ways to see what the overall effect is. Some choices are fabulous, and others are, shall we say, less than felicitous...