If you don’t have any requirements for the PDF/A-2b standard, you should be good by just using 1.7, which is also the default.
PDF/A-2b is in fact based on PDF 1.7, as the latter only specifies the version of the PDF, while the former also limits what functionality of the PDF specification is allowed to be used and enforces it more strictly. Many document archives thus require the use of the PDF/A-2b standard to guarantee that the PDF file remains readable in the future.