A little off topic, but consider:
1. donde, adonde/a donde
2. dónde, adónde/a dónde
and for that matter (or maybe especially for the case of),
3. quizá, quizás
Within each group the various options can be used interchangeably. But what factors influence the chosen form? For example do some people tend to use the same form all the time? Do people just randomly use all the forms equally? Do some localities tend to use one form more than others? Is there a pronunciation efficiency issue (similar to y and i or o and u, but not a hard-and-fast rule)?
I guess my questions especially apply to quizá versus quizás.