I cannot find anywhere it is explicitly stated. However, the idea behind giving someone a choice of resistances implies that the highest one is relevant.
It is the spell's target's choice, whichever they want. They are not told if the save is different though.
The reason why it works this way, is because people's resistances may differ a lot from their other resistances, and represent different ways that people can overcome the magic of the spell... Either through force of will, or their soul itself, or maybe through raw physical power.