picasa,
This really depends on the exact construction of your vessel. For instance, if you have common bolting holding the tubesheet to the shell and the end caps to the tubesheet, then you need to consider both the tubesheet pressure and the shell pressure as both of these resultant forces will act to separate the joint.
I would suggest that you do a simple free-body diagram of the forces on the joint. Do the forces add up or subtract from each other. I do not believe that this is a pure issue of either/or. It is the combined force on the joint.
If you have questions on the calculations, contact the code specialist with your ASME AI. I have used several in the past and always gotten some very good guidance. However, remember that the liability for the calculations lies solely with you and your company.