I'm not sure why you need a space behind the tank if all the plumbing is running through the wall?
I thinking, how much is the price difference between the bit bigger and 15mm and the 12mm? This tank would, hopefully, be in operation for 5-10 years and you're probably talking a running cost of $50-100/month. So we're talking up to $12,000 just to keep the lights on. What's an extra bit of money to begin with? Might as well go for as big and nice as possible to squeeze everything you can out of that running cost.