The 354 should give you a longer burn time at, say 24,000 BTU output than the 254 will at 24,000 BTU output. The maximum output for the 254 is about 50,000 BTU and for the 354 is about 65 BTU. The firebox in the 354 ( 74,602 c.i.) is wider, deeper and higher than the 254 ( 4,565 c.i.) so of course it will hold more coal and at a given heat output, last longer.