I literally squealed when I saw this because I'm the toy expert and this question seemed perfect for me!! They are both excellent toys but I rate Myla slightly higher. here's why!
Myla is a singing and styling toy with electronic functions but she has some cool coloring ability and a color sensing 'Magic' brush and palette which you can use to change the color of her wings, horn, eyeshadow, and mane! She just has a great look to her and stands at a nice impressive size. I also like her packaging more and think she makes an impressive present with game functions and the singing aspect. It's made by Vtech which is a great company that makes some fantastic electronic toys.
Rainglow is a vet set which means she has two modes. In one she's always well and will make noises and respond to stimuli. In the other she gets sick and through various tools the child treats her and makes her better... until a few moments later what she gets sick again. XD There is great play here with facial movements and things... but it's no Myla. I find that the Little Live Pets stuff tends to have limited play that repeats a lot.
That said the Rainglow can be found cheaper than Myla so if cost is a concern it's still a great toy (It's on sale on
Amazon) but I personally would go with Myla. I hope this helps!