If your goal is to get stronger, use the Prowler for strength work, not conditioning. This would be done for a 6 week program. Then you would choose a less intensive method of conditioning (AirDyne is the only thing I recommend besides the XVest).
If your goal is to just train, and get in good shape (or whatever) - just do them whenever as it doesn't matter too much.
The key is knowing EXACTLY what your goals are and program the training appropriately.