Why not a reg and BC next? If you have already completed your pool, training dives and have some open water experience then you may have had the opportunity to try out different types of equipment. There are many variations: in a reg it could be adjustable, non-adjustable, environmentally sealed (good for cold water) balanced or unbalanced, etc. The same with a BC- there are jackets, back inflates, hybrids and backplate/wing configurations. Each type has it's strengths and you will find people who will say "I only dive xxxxx".
In my limited experience, rental BCs are all jacket style. Some will have weight pockets, some will require the use of a weight belt. If you are able to find a place that will rent you a wing and back plate, I would definitely try that before spending money on a BC.
I mode the decision to stop renting and buy my own kit when I booked a trip to Cuba. I had heard excellent things about the reefs and horror stories about the gear. I had also heard excellent things about diving a wing instead of jacket, but I literally had to take it on faith. If you can try a wing for yourself, you won't have to take anyone's word for whether it is better or worse than a jacket.
between regs and a BC... weird decisions. I believe a rental BC will be safe, but each BC fits and trims differently, so if you want to work on having excellent horizontal trim in the water, you absolutely positively have to dive the same BC every time. So that's a vote for buying your own jacket BC or wing.
On the other hadn, sure regs are different but breathing is breathing and you'll do just fine diving different regs . However, you trust your life to a reg in a much more serious way than trusting your life to a BC. Let me put it this way: Which catastrophic failure would you prefer:
- Catastropic BC failure, but regs are working perfectly. You'll need to swim to the surface and/or ditch weights to survive.
- Catastrophic regulator falure. You'll need to locate your buddy, hope his octo works, and share air to survive.
Of course, the answer ought to be that you can handle either with ease. You have a balanced rig or ditch-able weights or a backup buoyancy system like a lift bag. You have a good buddy who keeps her kit in good working order and isn't dragging her octo through the silt.
But thinking about the choice does emphasize that having regs you take in for service and maintain imparts a huge feeling of comfort. While having a BC you swim all the time will help you learn more quickly.
Good luck and enjoy the ride.