Personally, I'd keep it really simple. Are they starting a new projects? SQL Server 2014. If they're going to go and purchase brand new software, right now, they might as well receive the full product life of that software. Suggesting 2008/R2/2012 for new development is negligent.
Now, past that, you need to determine if they have a need for Enterprise or Standard. In that case, you're going to have to ask a series of questions targeted around the differences between the two. The main areas of focus will be scale, high availability, and special needs such as change data capture, things like that.