I would go with 5.1.2 if it's a new install for the reasons you have listed. 5.3 will have some growing pains since it is a brand new version.
I had the same questions and decided that I have to go to ePO 5.30. If you are using McAfee Drive Encryption, then don't go to 5.12 as it breaks the ability to add encryption users. This has been documented by McAfee with no workaround. Since we use Drive Encryption my only option is ePO 5.30. I was told in the past that the only difference is that 5.30 will save extensions that you remove.