We've had a very good experience with Yamaha and if I had to go back, I would do it again. I love, love, love how they teach solfege singing like LMs, and the class is very child friendly. DD is finishing up Level 2 now at age 4.5. However, we skipped Music Wonderland entirely. I just didn't want to pay money for it when DD was already reading simple music, had advanced pitch skills, etc. Going to the sample class, I didn't think it was very different that other Mommy and Me music classes we had attended prior to that. But if you intend to stick with Yamaha for the long haul, it will be a nice introduction. Then, if you want to teach her note reading at home, you can do that early on your own. I really like that Yamaha has student kits with matching CDs, DVDs, Song Books, and Work Books, because layering the learning works very well for my kids. It's a very complete system that addresses all areas. They even send home parent tip sheets every week on practicing and connecting with your child and so on.
I can't speak for the Christofori classes at all, but the fact that Yamaha uses solfege versus the alphabet really drew me in. My kids can flip back and forth and are "bilingual" between do re mi/ C D E, but they are much more comfortable with solfege. I am grateful for that, because now my DD can repeat back so many songs in solfege and composes her own little ditties using colored notes a la LMS, and it provides a depth and fluency that the note alphabet cannot (in my non-professional opinion). My son tries to do so as well.
Since DD was already in class, I began teaching DS her songs at home and he practices with her still a certain amount. He is not old enough for Junior Music Level 1 but can play all of the songs in the book one handed and some with two hands. However, according to Yamaha he is only old enough for Music Wonderland, so I do think the age requirement hold them back a bit. Why don't you do a trial class at both and see which one you like better?
If you decide to go with Yamaha, you can use Suzie's Piano Primer DVD if you want to teach alphabet too, Little Musician of course (solfege), and Your Child Can Discover (alphabet) to begin to introduce the notes on a staff. And even Trebellina too.
Have you looked up some sample classes on you tube to get a feel for both programs? Also, have you met with both teachers to see how your child responds? Having a great teacher is probably more important than the branded delivery method, so don't discount your gut feeling when you meet the teacher!