Jamie,
The best thing to do is to download the README for MP5 and check to see if any of the fixes that are in it are needed for your site. You know your site better than anyone in this forum.
As far as stability, it is a toss up. I am doing fine with my 5.1 MP4, others are doing fine with their 5.1 MP5. Again, different environments.
symantec will always tell you to keep at the latest level.......