I would go for Regine. As what has been said, she has the range and the power. Add to this the uniqueness she gives her version of other people's songs. You would want to listen her finish a song because you wouldn't know how she'll attack it. That has kept her interesting up to now. She can bring songs to a higher level.
I also like Lea. I have followed her carerr since here "I am but a small voice" days. But singing seasoned by the theater is different from pop/mainstream singing. Her renditions are so predictable. Sometimes bordering on monotonous. I remember she did versions of "Sana Maulit Muli" and "Say That You Love Me", both songs having been (re)recorded by Regine already. She tried to give the songs the Lea-touch but she fell short in getting the audiences' nod. In the latter song she tried to hit the high notes and I truly pitied her. She should have stuck with her style so as not to look/sound TH (trying hard). I am proud of her awards and other achievements. But I guess when she is in Manila, she has to contend with the fact that Regine is the Philippines' frontliner, the country's premier singer.
Another thing going for Regine is that she doesn't pretend to be this "probinsyana before, sosyal na ngayon". he remains to be well-rooted on the ground. Even if the concert venue is the ballroom of a 5-star hotel, she won't try to pretend that she is as social as her audience. Lalabas ang pagiging donw to earth nya. To some it would be baduy. But that makes her tick! Kaya pa rin siyang abutin ng mga tao kaya tinatangkilik pa rin siya up to now.
I just saw over the TV her concert at the Westin, Songbird Sings the Classics. Her dress was so revealing but hindi malaswa. And my, oh my, I heard her sing "What King of Fool.." again and I said to myself, not even Mariah or Whitney can top that rendition. Gerard Salonga was her musical director. Baka nagsawa na sa ate nya.