Is this meant for the homebrew forum or are you asking what's better between +1 skill rank or +1 HP?
If homebrew, here's a few ideas:
The spiritualist’s phantom gains +1/4 HD (Max +5). This does not allow your phantom to gain special abilities earlier, but affects everything else related to HD as normal.
Add +1/5 to the bonus provided from Spiritual Interference. You can take this FCB before you gain the class feature.
Add +1 bonus on checks made by skills you have Skill Focus with. This option provides no benefit when your phantom is manifested, and you must be capable of manifesting a phantom to take this option.
Some of the other drow FCBs are about getting off-list spells related to darkness, evil etc. IIRC.