Do I have a base option when switching from guard to active duty or is it a random location?

When transferring from Guard or Reserve, and you don't get it in your contract for choice of assignment, then you'll be given a "dream sheet" to write down places you'd like to go (3 Stateside and 1-2 overseas). The Amry will try to match, but it's still gonna be based on their needs.

They may ask for preference but it's up to the needs of the military most of the time from what I have seen.

“Needs of the Army” comes to mind