For what it's worth, I just spoke to ScubaVentures in Bir
You don't know what you don't know, that's how your wrong (BTW you said that yourself). You are putting the cart before horse, worrying about equipment configurations when you haven't taken a single breathe on SCUBA. Its like you decided you want to learn to fly, and want skip to instrumentation panel layout in a GS650, never having even sat in Cessna 172, never mind flown one. You can watch 10,000 hours of youtube, and read every post ever written in Scubaboard, its all mute until you actually get in the water.
As far as bp/w versus jacket bcd, you are getting into an Apple vs. Windows, Betamax vs. VHS, and Dodge Truck vs. Ford Truck argument. And you have come down on the bp/w bcd side of things because joe shmoe on scubaboard has made up his mind about it, instead of making up your own mind about it.
Your two comparisons don't really align, namely IFR in a GS650 compared to basic lessons in a Cessna isn't the same as Ford vs Chevy, unless you're talking lap records in a Ford GT40 at Le Mans vs a hauling hay in a Chevy pickup on a Saturday afternoon.
In any case, how about we use an analogy that actually relates to me, namely motorcycles? I've been riding regularly for over 15 years, I have easily 100K+ road commuting miles under my belt in Seattle traffic year round, and even more if you count recreational rides. I've seen and ridden it all, downpours, snow, hail, steep uphills in stop-and-go, slippery old street downhills, manhole covers and trolley tracks in the rain, etc., so I'd argue I'm very experienced. Before I bought my first bike, I did a ton of research (that's my nature) and had my mind made up about what bike I wanted, but then I listened to the "experienced sales guy" and bought the bike he recommended (that his shop happened to carry). It was good, but I eventually sold it and bought the bike I actually wanted and realized that I would have been better off buying it in the first place. Not only because it was more aligned with my riding style, but also because it was easier to learn on despite what the sales guy and some misinformed people on the internet said. The two bikes were similar, both had two wheels, handlebars, etc., but there were differences in weight, balance and handling. The cumulative opinion of the owners of the bike I should have bought were correct all along because they owned the bike and had experience with it. That's why I originally listened to them.
Despite numerous personal and situational differences, people tend to come to accurate conclusions when you consider a large enough sample set, you just have to be willing to spend the time to research the topic, as well as topics related to it, to see all angles. You also have to be intelligent enough to filter out the trash. To that point, most people here tend to agree that a BP/W setup is more stable in the water, simpler (less failure points), obviously more modular, and thus more configurable to specific tasks. You see plenty reef diving in a BP/W, but show me people cave diving in a jacket BCD. Maybe there are and I just didn't find those examples, but I have a feeling there aren't many. Why? For the reasons I just mentioned, and those are the reasons that attract me to the BP/W configuration. If it's really Ford vs Chevy, then why do you and the instructor care which I choose? I see them as significantly different, though.
I bet dollars to donuts the shop owner doesn't get his hand smacked by PADI.
And that will say more about PADI than about him.