Answer

If there is more than one eligible hex for placing a Garrison, Skeleton, Legion, or Horde as a result of an effect, the acting player makes the final placement decision.

This is supported by the following rule:

• "Sometimes, the Event or the hex will tell you exactly where to place them. But in some cases, they will tell you to simply 'place 1 Horde' for example, allowing the acting player to choose."
• "If you cannot follow the instructions of a place effect, or there are no instructions, place the Unit on either an empty hex or a hex with Units of the same faction."
  (page 50)

Source

Page 50: Placing Nemesis Units

This section explains that when the placement is not strictly dictated by the effect, the acting player chooses among the eligible options.

Source: Core Rulebook (pp. 21, 34, 50, 51, 53)