You can view it in simple terms and just say that a plate is a plate, and a wing is a wing.
Or you can elaborate on it and complicate it further, and break it down further.
Plates: They can be stainless steel, aluminum, or kydex with stainless steel reinforcements. The plates will come in different sizes, and the different manufacturers have different bend angles. They are all shaped slightly different.
The bend angle will be most noticeable on on a singles rig. For every diver that recommends a flatter plate because it puts the tank closer to your back, there will be those that will complain that the 1st stage hits them in the back of the head.
Another choice is whether or not to purchase a design that requires a single tank adapter (STA). There are rigs that do not require a STA.
Some companies offer bolt on weight plates for their back plates, other companies offer weighted STA's.
Other companies, like "Fred T" offer plates made from thicker metal than the standard 1/8".
Harness choices: You can get a plain webbing harness, or get one of the padded harnesses. Each has their fans and detractors. There are also some designs that have some means of adjusting the harness. Halcyon has one style, Deep Sea Supply (DSS) has another type that can be added to their stainless steel plates. Golem Gear offers a harness that incorporates a different style of shoulder harness buckle.
There were some guys that added some type of fastex buckle to their shoulder strap, but they all died. So no one is sure how well that worked out.
The hardware that comes on the harnesses from different manufactures varies in the thickness of metal used. Most people don't care...I like the attention to detail. I like the companies that use the much thicker metal in their buckle.
Wings: Get a single wing for single tanks, and a double wing for double tanks.
Wing size is most important, then shape. Some companies offer many shapes and sizes for wings. Other companies have just a few.
Inner bladder material differs with each manufacturer, as does the outer material used in the shell. Just search here for numerous threads on this.
Embroidered company names on the sides of the wings, red trim, blue trim, different colored wings, etc. Whatever floats your boat, there are plenty of choices.
Inflator hose length is another thing that's different between manufacturers as well. Pull dumps, no pull dumps, look at this too.
The OPV in many wings have been recalled in the past due to springs corroding. This has probably been corrected by most companies by now.
I own DSS, Oxycheq, and used to have a Dive Rite wing. I replaced the OPV on all of them with a DSS brand. Theirs is an improvement over the China ones that are on most of the other ones.
There's an over view. I personally prefer DSS, for a variety of reasons. There are numerous threads here that really dig into comparing all of the brands.
For every recommendation, you will read from someone else that doesn't like them.
Read through, and you'll see the good and the bad. The really good will start to stand out.
They are all good, except for the ones that suck.
Reading BP/W threads on Scuba board. Have fun...see ya next year!
-Mitch