This depends on how you define "better able to represent."

In one sense, legislatures are far and away better able to represent citizen interests because there are more legislators who can represent more different sections of the society.  In my state of Washington, for example, we have a huge geographic split between the richer, more urban west side of the state and the poorer, more agricultural east side.  Our legislature has representatives from both parts of the state who can make the needs of very different constituencies heard.  So, the fact that a legislature has many members means that the voices of more of the people can be heard.

In another sense, executive branches are able to represent citizens more effectively because they have more power.  If the governor believes strongly in something, that something is more likely to become policy than if my local representative believes in it.

So, if "better able to represent" means that more voices should be represented, the legislature is better able.  However, if it means that the interests of some citizens should be put into policy as efficiently and quickly as possible, the executive is better able.