I have hard well water, should I purchase a steel or cast iron oil boiler?

My well water is very hard and my oil boiler is old and in need of replacement. Does it make a difference if I purchase a new steel or cast iron boiler? I've read that hard water can damage certain oil boilers over time, but I cannot find any info on which material might hold up better with hard water. Eventually I plan on installing a water softener, but right now money is tight and I can only afford to replace the boiler for now. Thanks for your assistance.