Imported post
If you get a 15, you can buy a pony later, get redundancy but are limited to "standard" rec diving. If you are a newbie, you'll appreciate the extra gas in a 15.
If you get a 12, you can also buy a pony later and get redundany. BUT, if you get hooked, you can get another 12 later and strap it with the first to form a twinset. Now you can start looking at more challenging, "technical" stuff. And having even more gas if great! However, doing simple dives with twin 12s can be overkill - and they do weigh LOTS (~36Kg full).
You could do the same with a 10 - same as for the 12s, but you start with a smaller tin. As a newbie, you might get fed up being first back to the boat all the time with the limited gas. However, if you twin with another 10 you end up with a much more manageable twinset.
You could go straight to twin 7s. A great setup, redundancy, easy to lug about. But, less gas, and your initial outlay is going to be much higher, including extra training unless you've got an experienced mate. More gas would be useful as a newbie - you can get higher pressure 300bar 7's which would help though. And if you want to go technical, you've already got the experience of twins - and two stage cylinders if you go to a bigger twinset later.
It's a tough choice - made easier usually by the price. With little cash and no inclination to get techy, a 15 would be a good start. Very brief I'm afraid...