RoboHelp for Word is no longer available in either RoboHelp 11 or RoboHelp 2015. It's time to move on to RoboHTML. If you are planning on upgrading, I would suggest getting the latest version. WebHelp is improved in RoboHelp 2015 (new skins) and Adobe officially supports only the last version of the product. (Though the graciously also publish security fixes for RH11.)
But perhaps it is better to end with a question: What is the reason for this question? Normally, people would just pick the latest version. So I'm thinking that I'm missing something that could influence the answer or will get you a completely different answer altogether.
Just came to mind: You may want to check out the reviews and feature overviews of the different RH versions.