Talking about stainless steel vs poly tanks, then poly tanks have their own specialty and steel tanks have their own. The main thing here that matters is that what is being offered with poly or stainless steel tanks.
Talking about durability and longer lifespan then there is a myth that almost every poly water tank will brittle and break down after it comes in contact with the Sun. in the end this concept has been developed as we all know that polymer is really a plastic and we are familiar with the thing that how plastic decomposes and becomes powdery over time. This is same as saying that all steels will decay in moisture or in rainy weather or if they come in contact with water, in the end we have all seen steel rusting and decaying.
Just like chromium is added to stainless steel to give it an extra protection from oxidization and rusting, the same way UV stabilizers can be mixed up with polymers to keep it from being brittle and to save it from decaying. Well in real every single object has a finite life, but this is not a major weakness in UV stabilized polymer tank.
Aquaplate rainwater tanks are a type of steel tanks used especially to store water, but most stainless steel tanks don’t have the strength to bare the corrosion problems so if they come in contact with:
Salty sea water.High marine conditions.Chlorine water commonly found in pools.
Based on these factors, you can choose your next water tank.