Idina Menzel
While I enjoy her version, she's a bit pitchy in certain parts. Although at some parts she's very emotional and I like that, because she's very ominous like Elsa and I'm glad that I like her version.
Lea Salonga
Many of toi know that she was the chant voice of Mulan and Jasmine, when I heard her version. I thought that she sounds even better than Idina! I would not actually mind her voicing Elsa anyway.
But I guess that they don't want Elsa to sound like jasmin and Mulan!
Conclusion
While both of them are good singers, I would say that Lea is the best singer out of these two!