If you're only going to use one or the other, I would use:

 - Metallic pressurized water fittings: **dope** because it will lubricate more effectively without being damaged by hard threads and you're at less danger of breaking the fitting due to over tightening.
 - Plastic pressurized water: **tape** because it will slide equally effectively against plastics, and will not "over lubricate" lending to breaking the fittings. Also, certain dopes can react with certain plastics (though most are probably safe).
 - Metal/Plastic drains: **dope** because it will more readily fill gaps to "seal" if there is no pressure backing it. 
 - Air/Gas: **tape** (or *specialized* dope, or nothing at all, depending on the fitting) because you don't want to potentially spread oils or other dope through your appliances/tools.