I've been diving CCR for about 18 years so don't know what is available now. In the early 2000's in the UK then the most common 10's were the dumpy Faber 10's and they were horrible as a twinset. Euros are better proportioned but it doesn't look like there is much in it between 10's and 12's. 600mm vs 690mm. I think I'd live with whatever downside there is for the extra 4 litres of gas.
The other thing to consider is who you are diving with and what sort of dives you'll be doing. I know it's obvious but twin 10's are significantly less gas. You are always going to be the limiting factor if everyone else is using 12's and your gas consumptions are roughly the same. You'll also be cutting dives a lot shorter than you could be if you want to have enough reserve to get someone else to the surface while gas sharing.
If you ever intend doing trimix then I'd forget 10's and go for a minimum of 12's. I'd also forget 300 bar if you intend doing anything other than air.
The obvious answer is get a rebreather