Hi Daniel,

The basic difference in the two processes is that in RSW the intended contact between two surfaces are planar, whereas in RPW a point contact is made between the two components. A spot weld is made by concentrating electrical current through copper electrodes with a contact surface of about 1/4 inch diameter. This gives approximately 200,000 amps per square inch, and 10,000 amps is enough to raise the steel to its melting point. Projection welding is done by concentrating the current with protrusions on one of the parts. The copper electrodes are still used to supply the current, but are usually larger, and longer lasting.
The Ohio Nut and Bolt company has a lot of on line information about projection welding, and even offers some electrodes that look like they may fit on a regular spot welder's shanks. Here's the link:

http://www.on-b.com/

In addition to the information provided on the above site, my brief research indicated that RSW is pretty much limited to sheet metal (up to about 0.125" thick), however the RSW process is occasionally used to join steel plates up to 1/4" thick, but loading of the joints is limited. Where fasteners are involved, the RPW process is used, as the contact area is too large to concentrate the current. RPW can be used in sheet metal (0.02 to 0.125" thick) to control exact spot placement where close tolerances are involved. Where welds must be placed close together, they must be done all at once, and RPW often is used this way. Welds made within 2" of a previous weld lose a lot of current through the shunt path of the previous weld, which has only 1/20th of the resistance of the new welding site. There are minimum spot spacing rules, based on material thickness, stackup and coating. RPW is easier to maintain, and requires a lot less troubleshooting knowledge, although it does require careful attention to surfaces, fit up and alignment. In general, the RPW process can be used instead of spot welding to join small parts to each other or to larger parts. Lap joint designs for RPW are similar to those for RSW, but generally speaking, joint overlap and edge distances for RPW can be less than those for RSW. Longer weld times and lower amperages are used for RPW than those for RSW. Spot welds have low tensile and fatigue strengths because of the notch around the periphery of the nugget between the sheets, and that the full strength of the sheet cannot be utilized across a spot welded joint because fusion is intermittent and loading is eccentric due to the overlap. Also, with sheet metal, the RPW process is limited to thicknesses in which acceptable projections can be formed and for which suitable welding equipment is available. In The Welding Handbook, Volume Three, Seventh Edition, there's a chapter that is devoted to the resistance welding processes and it covers everything. If you have access to a copy, you should be able find what you're looking for.