Well, there are lots of different straightedges out there, as you said, all designed for specific tasks. A wooden or metal foot rule and yard stick is a good place to start, for measuring stuff around the house. They will test for straight as well, within their limitations. For the shop, a quality combination square with the rule that comes with it is a good place to start. It is heavily multipurpose, and various attachments and longer and shorter rules extend the usefulness of the set. More specialized tools, like precision straightedges and ones for scraping in flat surfaces, are for tasks that most of us will not be working on. It is far easier to recommend a straightedge for a given task or next project than guess what you might be building in the future. When you have a measuring task coming up that you are not sure you know how to handle, ask the specific question you need an answer to. Sorry, don't know what else to tell you...