This is not something we can usefully do for you. You have the knowledge of what your users need and would like, how you've got your 4.x system set up and and so-on.
Any comparison we might do would be biased by our opinions and experiences and it would take us a considerable amount of time.
You really should do this work for yourself - as Peter says, the release notes are by far the best place to start, although I would recommend reading all of them - 4.3, 4.4, 5.0, 5.1 and 6.0 (The 'point' releases are almost always bug fixes - I wouldn't bother with them until you've decided which mid-level version you would like - e.g. decide you want to move to 6.0 first, then have a quick read of 6.0.1, 6.0.2 and 6.0.3 to see if there are any specific bugs/issues fixed in there that might affect you)
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