[Gareth Burrows]

deco cylinder and a pony behind you?

This means that when swtiching regs, you cant vizually identify which one of the cylinders will allow you to decompress safely, and which one won't. Or alternatively which one is safe to breathe at the bottom in an emergency, and which one will probably kill you. Your buddies, who face the same decisions, would be in same position. Everyone is now dependant on the regs being precisely where they are supposed to be. The problem is that in an emegercy, things tend not to be.

what could possibly go wrong.

Do the course, listen to the instructor, and then make the purchase decisions. Any decent instructor will allow you to borrow/ try out different bottles anyway, and there are pros and cons of each bottle type and size that should be explained on the course.