Plates vary in material (SS vs Al, and type of SS), quality of finish, placement and number of slots and holes, and angle of bend. Deep Sea Supply's plates also have an elastomer insert in the lower harness slots, to make adjusting the harness easier, and to cut down on abrasion. My husband dives a DSS SS plate and loves it. He also loves the weight plates Tobin makes for his setups, which allow him to put 8 more pounds on his back in a very streamlined fashion. (These weight plates are not compatible with, or available for any other plate that I know of.) He bought a Dive Rite plate when we were in Florida, but it put the harness straps in a different place, and after a few dives, he sold it.
Harnesses come in two flavors: simple Hog harnesses, and fancy harnesses. The simple harness is a length of 2" webbing, onto which you thread triglides and D rings and position them appropriately. The fancy harnesses may have padding, more D-rings, quick releases, and the like. Some people like them and don't mind the additional outlay; everyone I know who began with a fancy harness has converted to a simple one over time, just because they're more flexible in terms of being able to adjust them perfectly and position all the hardware exactly where you want it.
The only minor point I'll mention about DSS stainless plates is that the angle of bend is optimized for single tank use. Some sets of doubles don't have bolts long enough to use with DSS plates. If you aren't diving doubles, this is immaterial to you.