Fixed base is a base which resists Moments along with Tension and shear whereas Pinned base resists tension and shear.
In reality the action is always between the two i.e niether does a FIXED BASE nor PINNED BASE exists in reality.
Coming to your situation, the way you design & detail your baseplate and foundation will define whether you should use a fixed base or pinned base.
If you design the moment to be transfered through your baseplate to the fundation then it is a fixed base and if you only take tension and shear transfered to the foundation it would be a pinned base.
Hope that clarifies you doubt.