Zacapa and El Dorado are pretty different. Zacapa ages in a Solera system (most commonly used to age Sherry). Basically, you take a barrel of rum, age it, pull out half of it, put that in a new barrel, and add new rum to the first barrel. Further age that first barrel, take out half, add new rum, etc etc and so on. It's like a constant blend/age process.
El Dorado meanwhile is an "English style" rum, meaning they distill with both copper pot stills and modern column stills, and then blend those together and age in used bourbon casks. Using the two types of stills results in a rum that is considerably less "sweet" than "Spanish style" rums like Zacapa, Zaya, Diplomatico, etc. El Do 15 is stil definitely rummy, but it's one I often pour for whiskey drinkers or those who otherwise claim they "don't like rum."