Currently there are no security issues with Atahualpa that 'I' know of, and never have been. If you are happy, you can stay with 3.4.9 - you will miss out on the export of the settings.
If you switch to 3.5.3 from 3.4.9, your look should not change since it will be using the same options in both cases. By installing both in the themes folder, you can swap back and forth.
If you do a NEW install, you will see a different look, it is one of the new styles that are shipped as a starting point - others including the classic style, are included in the styles folder.