Hi videogirl,
everyone has an opinion based on their experiences and or agency, DIR advocates tend to have the same one
I would go on the course and let your instructor show you, and let you try different configurations and tell you of their pros and cons, I did my BSAC advanced nitrox with a 15lt and 3lt backmounted pony, the same year I did my IANTD advanced nitrox with independent twin 10's and a stage rigged 3lt, I did my ERD with inverted twin 12's and a 7lt stage.
I was lucky that none of my instructors had closed minds.
With the faceless media that is the internet it's easy to make a 'wrong for you' decision based on another's experience, so my advice is to make your mind up as to what configuration is best for you when you have tried as many setups has you can.
Safe diving,
Steve