Both are phonics based programs. I feel WRTR is much better. My oldest got bogged down around the 50th lesson in 100 Easy Lessons. What I like about WRTR is that it is a Spelling, Penmanship and Reading program all in one. Spalding uses the Extended Ayres List as soon as you've learned the first round of basic phonograms.
Spelling is reinforced and practiced to mastery.
The precision and exactness of the WRTR program gives the child the sense that precision and exactness is required in spelling as well. If you are schooling classically I feel WRTR is an excellent addition. Right from the beginning an exact method for forming your letters is taught. Learning to mastery is stressed throughout, no guesswork allowed. Writing the phonograms and Ayres' spelling words reinforces the visual and kinesthetic ways of learning.