Tarja has a great voice, but her range and, more importantly, her tone, are very limited. I loved Nightwish for the first five albums, and while part of that was because of Tarja's powerful vocals, the simple fact is that her musical comfort zone seems to be rather small. Now ironically, while Epica is not quite as diverse as Nightwish, Simone is a far more versatile singer. She has a multitude of tones that she can draw upon to add to the flavour of whatever she is singing, going from soft and gentle, to powerful and awe-inspiring, to dark and sultry, to up-beat and poppy and many more besides. Tarja really just had the high-end operatic vocal thing going for her. Her attempts at different tones and styles that break out of her operatic box are not really that impressive. Simone can not only do the opera thing, but she can also branch out a bit more into rock/metal and even more pop-infused styles with her voice. And it also seems like Simone puts a bit more emotion and feeling into her vocals, while a lot of Tarja's songs just sound like operatic masterclass exercises. I also believe (although I may be wrong) that Simone also has a much wider range in terms of octaves, as well as a broader range of tones.
So yeah, Simone, by far.