We can go to the seven o'clock performance or the eight – whichever works best.

whichever 是疑问代词 which（哪一个）加上后缀 -ever 构成的复合词，表示"无论哪一个"或"随便哪一个"。在句中引导选择关系，强调从多个选项中任选其一都可以。