Five tips:
a) Be certain when you compare costs that each kit is complete. What this means is: backplate, wing, hog harness (resin impregnated webbing, buckle, d-rings, tri-glides), inflator hose, two cam straps/bands, crotch strap and ideally some little blocks that help keep the tank snug (elastomer blocks). You may need a STA although one is not needed with DSS or some Oxycheq wings.
b) Get it as far in advance as possible. Spend some time adjusting the webbing and getting things just so in your living room. Taking it off and on. No tank or regs -- just the plate and harness. Note: when in the water, the shoulder straps will be looser than you thought and the crotch strap will seem tighter than you thought.
c) Head to a pool if you can. More tweaking at this stage. It will soon be dialed in and it will fit better than any BC has ever fit.
d) Get the right size wing. Too big means too much drag. Do not get a wing that someone says is better because it work on either singles or doubles or that has tons of lift.
e) I like DSS a great deal (own wing, plate and harness) and the Oxycheq wings are nice as well (own a wing). Many other brands are solid.
Why are many suggesting DSS?
a) Lots of size options on the plate as well coming in heavy (stainless) or light (Kydex). Lots of wing options as well.
b) Very solid and well thought out products.
c) Great customer service. This includes working hard to understand what product (including wing size) they think will work best for you. They don't try and sell you extras (in fact the opposite).
d) Their complete rig is really well made, complete and a solid value at around $460 (depending on choices) plus shipping. (That includes a stainless plate, wing, two cam straps and wedge blocks and the harness. The harness kit is 12 ft of webbing, DIR style knife and sheath, buckle, some cave line, 5 d rings, crotch strap, 5 tri glides, and some inner tube pieces). Add a marine bolt snap (and attach it to your gauges with the cave line) and you have a pretty fine kit.
I have also used Cave Adventurers (for the Oxy wing) and they were solid as well. In the end, I rate my DSS BP/W as one of my two best gear purchases ever (the other being a DUI classic weight belt harness).