Version 4.12 is a more incremental update of smaller features. Both 4.12 and 5.0 should include the same bug fixes so no major differences there.
Version 5.0 is a big upgrade and substantially changes a couple of important areas - CRM and Taxation. If you are interested in enhanced CRM functionality or interested in integration with the Avalara taxation service then version 5.0 is the right option, otherwise you can take the more conservative approach and upgrade to 4.12.
At some stage the 4.12 path will end and you will have to upgrade to the 5.x path, but at this stage there are two options for you, and the correct choice depends on your type of business, and the impact that a major upgrade might have on that business.
Whatever your choice, make sure you back up your database first, do a mock upgrade and then test all of your important transactions and business processes. If you find any issues, please assist us by raising an bug in the Issue register for us to investigate and resolve (based on priorities of course). When you are comfortable that all processes are working you can consider an actual upgrade of your production system.