Thanks for the question! Pausing a workbook keeps all your tables in memory, while deactivating a workbook releases all your tables and frees up the memory they were using. The advantage of pausing your workbooks is that reactivating them will be very fast, because the tables don't have to be reconstituted by replaying the operations you've performed in the workbook. The disadvantage is that you're squatting on memory that other cluster users might need. So if you're packing your bags for vacation, the friendly thing to do for your co-workers is to deactivate your workbook. If you just want to visit another workbook and then come back to the current one, pausing might be a good option, especially if the cluster has plenty of memory.
Note that if you activate workbook B when workbook A is still active, workbook A will be automatically deactivated. When you go back to workbook A, you'll have to wait for a full replay before you can use it again. If you don't want this to happen, be sure to explicitly pause workbook A before activating workbook B.
Of course, no matter whether you deactivate or pause, you won't ever lose your data or your dataflow.