I'll go with Tiffany Mynx, because of a past experiance at the O'Farrell Theater in San Francisco in my early 20's. I just wish on her current stuff she'd stop taking 3 in the ass at the same time.
Alexis Texas has a bigger ass, but she's a mediocre performer. I would have to watch more scenes of Tiffany Mynx to be able to say which one is better overall.