Thanks for the email, and glad you're having success with Spark on so many projects. I do actively maintain Spark and I fix any bugs that come in, but there is not much changing in the world of Spark. Personally I don't use MVC anymore since moving everything over to frameworks like Fubu, ServiceStack and Nancy depending on the client and requirements so I can't say I've ever done anything on MVC5 - the last version I worked on is MVC4. There should be no problem with Spark working with MVC5 however, and the latest Nuget has got binaries all the way from .Net 2 up to .Net 4.5 packaged inside. If someone wants to test it out on MVC5 and let me know if there are any bugs, I'll be happy to release a Nuget for it after fixing any bugs.
FWIW, the best source (because it's the easiest to deploy to) will always be Nuget unless that dies.