I think you are talking about a water heater, if so 12-15 years is a very good life for water heater. So if you can spend 300 and get the chance of 6 or so good years, or even just a few years, than it might be worth it. But remember, no warranty, no guarantee to last 6 years, no recourse if something goes wrong.
Do you have to go with an oil water heater or can you go with an indirect??