The written text of constitution and subordinate legislation is the law of land. When the law is silent, convention and usage comes in play. However, it may be added that law is framed after considering the social practices and conventions.
Convention is a rule But it is not enforceable by courts.
Constitution is enforceable by courts. And It is necessary for constitutional government.
I think Both are equally important.
Definitely rules, rules are what keep an entity in control.Covnentions can arise out of sentiments and radicalism ,thats why rules are to help in high regard to prevent chaos and anarchy.
I thought they were inter connected, Rules are important and conventions form part of the rule since one cannot have rules unless conventions are taken into account !