Mello would have won.
Today I actually found said interview. Which shocked me. I secretly thought a Mello fan had made it up!
Here is the transcript of the relevant section:
Were Mello’s actions at the end already decided from the start?
No, it was not. I was still pondering about what to do with Mello when the story reached the stage where Shidoh returned to Death Gods’ world. I had intended that Mello would be the one to defeat Light. Being number 2 at Wammy’s house, Mello was the dark horse whom I wanted to see the winner.
Why was this not the ending that was told?
Mello knew too much about Death Note. If he had won, it would be too easy a win. Hence, I made him attempt to physically capture Light one on one, resulting in his death, which was meant to be very dramatic.
Original interview with Tsugumi Ohba by The Star Online, but no cyber version exists. English translation by ScarredinSilence