The Kerbodyne S3-14400 fuel tank's statistic is just the double in anything I'm considering so far of the Kerbodyne S3-7200 Tank. It costs twice as much money, is twice as high, contains twice as much fuel. So it seems (to me) to be the same as just connectiong 2 Kerbodyne S3-7200 Tank with each other.
The only difference I can see, is:
It is an additional node of the tech tree requiring 500 science to invest for unlocking as the highend invention of fuel tanks, while the S3-7200 Tank is an inexchangable prerequirement of it. So why I should ever spent 500 science to receive a part, I'm unavoidable allready able to exactly build my self of preavailable parts (namely: 2x Kerbodyne S3-7200 Tank)?
Best Answer
Substituting two smaller fuel tanks with a larger one reduces the part count of your vessel. This can matter when you haven't yet fully upgraded your VAB and SPH.
A lower part count also improves game performance and prevents physics bugs which come from too many parts and lead to unexplained wobbling and explosions. Less connections also leads to more overall stability of your vessel.
But I still wouldn't consider that tech node a priority. There are far more useful tech nodes in that price segment.