In my opinion, A& = B& will be saying that B's 4 byte long integer register's data will be copied into A's 4 byte long integer register. Saying A = B on the other hand will be saying B's unknown register's data will be copied into A's unknown register. And extra assembly code will be added to convert them to their proper data types, thus slowing things down.
If you are using Option Explicit and have declared A As Long and B As Long, no data type conversion will be done. Compiler is smart enough to optimize that.