My wife and I got certified in jacket BCs. She's small, and the smallest one never fit her right. Mine fit, but I found I would stop moving, and all the air inside would keep moving, making it hard to get properly trimmed. There's typically a lot of fabric on jacket BCs, which has a little buoyancy and traps tiny air bubbles, so right off the bat you'll need more weight compared to a backplate setup with just a harness and wing (someone told me a Zeagle Ranger is something like 8 lbs positively buoyant just because of all the fabric, not sure if I believe that or not). From my own experience, if you go from a jacket BC to a 6 lb stainless backplate, you'll be able to take off a bit more than 6 lbs of lead once you get things figured out after a few dives
I have both continuous webbing harnesses and comfortable harnesses with padded shoulders and quick release buckles. To each his own, but I find the continuous webbing less comfortable, particularly for a long walk for a beach dive, even with a 7mm wetsuit and hooded vest. In the water, you won't feel any difference. And if you have different thickness suits, or go from wetsuit to drysuit, a comfortable harness like the DiveRite Transplate is definitely easier to deal with
I still have my jacket BC. I use it in the pool and don't really care what the chlorine does to it. It's pretty indestructible
My wife won't dive with anything except her backplate setup. Unlike jacket or back inflate BCs, you can size a backplate and harness exactly to fit you