That depends. Yes, ADF is a great idea, and can be a huge work-saver, and if you have the cash to spend, it may well be worth it. But ADF has a dirty little secret: like any other gizmo with moving parts, it occasionally jams. The sheetfeeders built into these low-cost machines are aimed at handling typical office paper straight from the laser printer--large, smooth, good quality, with perfectly-cut, perfectly-aligned edges. In your PG work, you will be dealing with hundred-year-old pages of various thicknesses and textures, usually much smaller than the sheetfeeder was designed to work with. And you will have to have cut the pages, and may leave ragged edges in doing so.
Under these conditions, you may find that paper often jams in your sheetfeeder, and it defeats the purpose if you have to stand over the scanner while it works, or if you end up having to lift the cover and use your scanner as an ordinary flatbed, or, worse, if your paper gets scrunched up as if a dog had been playing with it.
And of course, in order to feed the pages through, you will have to cut them out of the book, destroying it. (It may be possible, with the help of a bookbinder, to have the pages professionally cut, and later re-bound.)
With ADF, you probably won't actually scan much faster than scanning flat, but you won't have to keep turning over the pages during that time.
So when you're making that choice, think carefully. If money isn't a problem, or you do expect to be working with cut sheets, then go ahead and get a sheetfeeder--it's great when it works! But don't be disappointed when it doesn't work all the time.