You don't have to upgrade to .424 if you are enjoying .415 but you won't know what you are missing in the newer version. For some people it's better having the latest version which is also official. For others, it makes little difference. However, it's widely believed that the latest OS is usually better since it may have some improvements over the previous version and bug fixes. I have not noticed much difference between .415 and .424 except better stability with Dark Themes in Text Messages, Calendar and BBM but it is still not working for me in Contacts although it worked initially. Also, 424 seem to run a bit faster and smoother but it may be just the feeling one gets after upgrading to the latest official OS. You will have to muster your courage and take the plunge to test the water yourself. You can't soar with the eagles unless you are brave enough to fly with the flock.