1) Depends on how many chicken halves...
2) If just a couple, then I would use the kettle.
With an upper cooking grate (modified 18.5 grate) you could easily do more on the kettle.
I have an 18.5 WSM and a 22.5 kettle, so no favoritism.
I like the kettle for cooks like this and use the WSM for bigger cooks.