Shall you use one of these, according to [Oxford Dictionaries Online][1], you should only use an apostrophe "for the sake of clarity", therefore opting for the first option ($x_i$s).

There are cases, like [this][2] in Statistics, where you can use the plural, for example, for a variable as the *p* value. You would then write *p*s to show the use of multiple *p* values.

However, these are different variables, with possibly unrelated calculations (even though the formula is the same). In the case of a sequence, $x_i$ is already a generic term, representing each element or the sequence itself. You can then write:

 - "the sequence $x_i$", or
 - "the sequence $(x_i)$"

  [1]: http://www.oxforddictionaries.com/words/apostrophe
  [2]: http://blog.apastyle.org/apastyle/2011/08/the-grammar-of-mathematics-writing-about-variables-.html