If I'm allowed to cheat, then I choose:
1. Sisters of the Moon
2. Gypsy
But if I absolutely must go by the rules, then I guess I would choose:
Stevie: Sisters of the Moon
Lindsey: I'm So Afraid (with Stevie as back up, à la The Dance)
Sisters of the Moon is awesome, but I prefer the album version over the live interpretation. I much prefer to hear Stevie's howling "me" and "moon" rather than the shortened "me, yeah!" "moon, yeah!". Also, I think John's bass defines this song, and in the live performances I've seen, it was almost drowned out. I haven't heard it live in person though, so that could account for it.
I would be interested in hearing Stevie and Lindsey sing Christine's songs, but I would rather hear more of their own, rarely played songs.