Cutting dovetails is an old art. Despite the availability of fancy (and expensive) mechanical jigs, it’s hard to beat the satisfaction derived from using a marker and cutting the dovetails by hand with a backsaw. I’ve been using traditional dovetail markers like these for about 15 years. The actual ones I use were my grandfather’s. Because he passed away before I was born, I don’t know where they came from; but I’ve seen these markers in person and believe they’re of the same excellent quality.
The nice thing about fixed, brass markers like these is that they will (unless you’re really sloppy) be the same every time you trace with them. They’re made of a stalwart material that can suffer the slings and arrows of most workshops. Cheaper versions will work fine and all, but there’s more margin for error. Sliding bevels can be a precision tool — in the right hands. Because there are moving parts, it’s easy for things to go wrong (wing nuts are not the most secure mechanism). In my personal experience, I use the sliding bevel for non-precision work. Because you’re having to match up precision cuts in two disparate pieces of wood, anything other than an exact mark (and cut) is a disaster: if the dovetails are off even just a little, so is the whole joint.
I’ve fixed/rebuilt my great-great-grandmother’s 1860-something White Sewing Machine stand drawers using my grandfather’s hand tools — the dovetail markers, his planes, backsaw, etc. I couldn’t have rebuilt the drawers w/out the dovetail markers, because the drawers are approximately 5 inches high and only about 3.5 inches wide. The dovetails had to be marked and cut very precisely or the whole job would have been botched. I also credit the markers with my zero wood waste on the project.
I find that cutting dovetails by hand (as opposed to using a joiner – a machine) is actually faster, but like anything, it requires practice. It won’t be faster the first time. But if you take the care to do it correctly, it will always be better than a mechanical process. Honestly though, woodworking isn’t about saving time. If you want to save time, go to Ikea. If you want to create something really, really cool and know that your grandchildren can be told that grandma/pa made that with her/his very own hands, then you use high quality tools like these and play with making stuff out of wood. Made correctly, your dovetails will be accurate and the joint will last 100+ years.