Both have positives and negatives. A cast iron stove is bolted together and the joins sealed with fire cement or similar. This can deterioate over time, bascially creating weak spots within the body of the stove. Another negative is that cast iron can crack if the temperature rises too high. The main positive for a cast iron stove is that it will often hold its heat for longer than a steel one. – but then again, it will often take longer to heat up when you first light it.
Steel is more flexible – modern manufacturing methods allow a stove body to be formed into one piece, which means no weak spots. Steel will heat up quicker than cast and although it won’t retain the heat for as long after the fire has died down, it will still stay warm for many hours. Most of the top quality stove manufacturers are now making their stoves out of steel – it’s cheaper than cast too, so weighing up all the pros and cons, we think steel probably comes up trumps.