UOM Conversions & primary UOM setting

We are working on a 9.2 implementation.  We have a lot of discussion around UOM and what we will set as the primary UOM, and how we want to set UOM conversions.

We have some items that fall in the below scenario:

Most branch plants transact in CA

A few branch plants transact in EA

What are the downfalls of setting the primary UOM to CA, even though some branch plants may transact in EA?

We also want to restrict some branches from transacting in EA, so we would then be required to set the UOM conversions at the Item/Branch level.  The other option would be to set-up a new item for transacting in EA and extend that item only to the branch plants needed, then transfer from the CA item to the EA item.