You will be able to run both a 3.x website and a 6.0 on one server (if this is your concern). The difference will be in the IIS configurations, because for example 3.7 versions of Sitefinity that are using .Net 3.5 can be ran only on application pool with .Net 2.0, and the application pool or 6.0 website should be .net 4.0 Integrated. However, this should not be a problem for you. Do you have any other concerns?
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items