The major difference in the models is how much hot water you heat. So to heat more water you have a larger firebox. Having more hot water will provide you with more btu's of heat for your home. In theory if it takes so many btu's to heat your home then the units should perform similarly. The difference should be in longer burn times since you have a larger firebox and more hot water at your heating disposal.
In other words, if you have a 6048 that is putting out 300,000 btu/hr and a 5036 that puts out 200,000 btu/hr and your house requires 100,000 btu/hr then you can see that you'll use up more of the potential in the 5036 during one hour of heating. But then that's complicated by things like amount of wood in the box when it has to fire, what type of wood, etc. That's why the numbers they give aren't that accurate.
Go to heating help and get the free heat loss calculator from slant fin.