Heart of the Swarm Campaign- Umoja Conclusion

With the Dominion defeated here and the rendezvous point secure, Kerrigan is bereaved to find out from a galactic news network that Jim has been captured and killed by Arcturus Mengsk's forces.  This also spurs her need for vengeance upon the emperor, so she sets out to do just that.

You have two selections for where to go from there- Kaldir and Char.

Going to Kaldir will let you add both roaches and hydralisks to your available units list.  This is a very useful combination with the zerglings you also have access to, providing you with a stronger offensive ranged option than queens and heavier armor as well.  The versatility of these units is not to be underestimated- roaches' ability to regenerate reduces your need for queens to keep your forces healthy, and the hydralisks' stronger attack power makes them more effective at taking out air units, as well as complementing the roaches by having a longer range and letting you establish battle lines.  This is not a choice to take too lightly, though- so far you have been facing Terran forces, and Kaldir is a conflict with the Protoss.

Alternately, you can head to Char, the site of the end of the Wings of Liberty campaign.  By going there, you can have Sarah encounter General Warfield, alongside whom Jim fought during Wings of Liberty, and will continue to face the already familiar Terran forces.  You will also gain access to the mutation that lets zerglings turn into the suicidal banelings, and to aberrations.  While this will still leave your forces very vulnerable to anti-light-armor units, aberrations' lifestealing capability and massive health work together extremely well with the Transfusion ability that queens have, which is much stronger with a small number of large units than with a large number of small units.  Additionally, banelings are excellent for taking out small enemy units, which lets your aberrations attack only where they should- enemy buildings and heavy units.

At the moment, either option is equally good, both offering a set of useful combined units and a decent number of additional levels for Kerrigan.