As far as boiler operating efficiency the most efficient operation would be the modulating boiler. When the boiler is condensing you gain btu's from latent heat until the condensing mode is over. Now the efficiency drops quickly. Even though the efficiency drops it never drops below or in most cases as low as cast iron boilers. The most important thing to remember is proper sizing, installation and outdoor reset. Keep in mind that system efficiency is more important than boiler efficiency.
Let's compare these things also. Life cycle costs which include, installation cost, operation costs, maintenance costs and service costs.
When all things are compared there is not much difference between the two units over the life of the products. The cast iron boiler will last much longer but that may not be the best thing wither. I believe even though the boiler is still good 15 years from now, life expectancy of a mod/con boiler, you may still want to change out the cast iron boiler due to technological improvements.
If I was going to install a cast iron boiler I would be looking at cast iron boilers that are capable of returning cooler return water temperatures. Most cast iron equipment the minimum return temperature should be a minimum of 130f. There are other cast iron boilers that can return 100 - 110f which will increase system efficiency.