If you haven't come across this post yet about inlining changes in 3.5 SP1 it's a good read.  The short version is, value types will have better performance.

http://blogs.msdn.com/clrcodegeneration/archive/2007/11/02/how-are-value-types-implemented-in-the-32-bit-clr-what-has-been-done-to-improve-their-performance.aspx