Sion's Zeta- turns him into an absolute beast, as it's much harder to drop his taunt, and he gets to his buff faster
Basic Omega- depends on who your fighting. Some teams aren't debuff heavy, so it won't matter much. Against teams that are debuff heavy, shaking off the debuffs can make Sion hard to pin down. Sometimes other Zpalp teams benifit more from Sion cleansing than he does
Break Will Omega- Sometimes a resistance or jedi unit is within killing range, but has foresight. That's when your grateful for this omega. If your fighting neither team, then probably not
Sion v Tarkin- Tarkin applies more debuffs and follows that up w TMR, reinforced by his tenacity up and is quite awesome at getting your team ahead because of the debuffs and manipulation, Sion is a tank, so he'll soak damage (you may want to consider adding Shore or SiT once you get them, cuz he's not uber tough until he buffs), and he'll clear buffs with his aoe. Debuffing him will boost DN and if the enemy focus DN they'll boost Sion. Sion takes the cake in my eyes, but Tarkin is a worthy unit on a Palp team if that double skill dump can put them far enough ahead