Do both Talia and Spirit, but make Initiate a side project too. If you ever want to use them in the arena, it's nice to have the option of using either. I personally use Spirit in the arena unless I'm going against Nightmare (especially ones with a zSion). Talia's group cleanse is the only way to get around Sion's taunt. Everyone else, I use Spirit.
For HSTR you'll definitely want Talia for P4. In P3 Spirit is better than Talia and works well, but the double debuff attempts from Initiate's basic make her the preferred 5th for that phase.