i almost always go with synergy > diversity simply because it often results in a much more stable team. for example, heatran + celebi cover eachother's weaknesses perfectly, but the problem is that neither of them will be getting past dedicated special walls very soon. another example would be that rhyperior obviously resists fire, but it's abysmal spd doesn't allow it to consistently check heatran or chandelure. for these reasons and from my experience, it's always better to build off of what opposing pokemon you need to check / counter, rather than what types you need to have a resistance to.
xx10charxx