I need some help figuring out what type of water tank we should get installed.

We have a tankless boiler (Beckett) which runs off oil. It was installed in Fall 2009. We did not have any issue with the old boiler but since last winter evryone has been complining about not having enough hot water for shower. The water keeps turning cold to hot to cold every few minutes.

We talked to our oil supplier who also installed the boiler and he suggested to have a tank installed. He gave us two quotes,one for about $1500 and one for $2400. He said the more expensive one is made up stainless still and would last longer.

We do not know anything about the water tanks. Below is what we would like to know.

1) Are there any other options?
2) Would it be better to get electric heater added?
3) What kind of things we should consider besides financial burden?