Topic, I still see demand for CB's but it seems zerks are almost always used.

When would I use one over the other?

In pvp axe because it has the same range, higher damage, and lower reqs.

In pvm just use whatever looks coolest, I think most people would agree that CB > BA for looks. I'd go with GPA if using ebotd though, for pvm.

from a technical point of view ba is better in any aspect.
if you think style has a major role then follow you taste, this usually work for pvm or melee duels.

Why not war pyke

If you are talking about war pike, its not commonly used because its slow.

Only weapon IAS affects WW IAS speed. The GPA is a faster base weapon and you cannot make up the IAS difference elsewhere.

Why not equip school bus

Because the game will crash with kids inside.

always, because botd ba is always bad, while botd cb can be used for 2h ww