300 l or 500 l tank?

Another question we are having to replace a leaking hot water tank but we don't know whether to get a 300 or 500 l one.
We have an oil boiler that feeds the tank ( it has no element in it) and we have 5 showers and 1 bath in our bathroom ( we run a small apartment house in the Alps). The plumber says we need a 500 tank , at the moment we have a 300 and never seem to run out of hot water. My question is which would would be the most fuel efficient?
My thoughts are that it seems silly heating up 500 l of water when there is no -one else in the house apart from us just for one bath. The apartment are probably full for about 20 weeks of the year so for the rest it is just our needs.
Any thoughts.

Winter and Summer holidays in the alps!!!
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