I think a better question is what do you need and what is your current skillset. If you have some understanding of development and or content management architecture WordPress would be the better tool for you. If you have no background and don't have incredibly complex customization needs then Rainmaker would be a better fit. For someone with little cms background Rainmaker will win the ease of use and appearance, otherwise versatility, seo, scalability, and customization potential - WordPress would be your winner.

Disclosure: in the last 5 years my company has built 600+ WordPress sites - however we have used many of the top competing CMS platforms and have researched Rainmaker and other hosted CMS alternatives for our clients and implemented some of those alternatives.