I'd always go for the bigger tanks. Sure you waste some CO2 but you already plan to double batch into them. If your using them as true brite tanks and filtering into them and packaging quickly you can clean them with acid under pressure and go right back into them with minimal CO2 loss. Lots of breweries have multiple size fermenters and only partially fill brites at certain times. No worries on spoilage unless you are not clean to start with. Most brewpubs serve from brites and by the end you may have .5 -1 Bbl in a 30 Bbl brite.