Initiative, always initiative for spellcasters. The sooner you can get any spell off the better. You want to go first so that you can buff yourself, buff your allies, debuff the enemy, build a wall, create a pit trap, create fog, conjure a storm, summon a monster, or otherwise make a big impact before anyone else can act.
The reason for this is that usually at the start of combat your allies and/or the enemies will be in a small tight nit group perfect for your AoE buffs or debuffs. Each turn that passes before yours is one less target to be affected by your spell. Your effectiveness in round one is highly impacted by your initiative result. So every extra point you can get to initiative you want. Also, in the event that combat starts with everyone scattered, the sooner you can cast spells, the sooner you can manipulate the field to funnel enemies into one spot with walls and pits. And if you’re just the blasty blaster type, again, odds are they will be close together at start of initiative and every turn before you is one more enemy out of range of that empowered maximized fireball.
Your will saves will be fairly well off from your high will bonus due to casting class.