You say you want to supplement your flatbed scanner with a document scanner to allow you to quickly input the genealogical media you get from relatives and other sources.
I say don't use a document scanner. Instead use a digital camera. If you have a reasonably new one, it has a macro mode and should have the resolution you want. 300 dpi for 8.5 x 11 paper is 2550 x 3300 pixels and needs about an 8 MP (megapixel) camera.
The newer smartphones such as the iPhone 5 and the Nokia Lumia 920 have incredible digital cameras that will serve the purpose just as well. How much more portable can you get than the camera or phone in your pocket or purse? These also can do face detection, image editing, can upload your images to your cloud drive, and even have scanning apps. And you can also use your smartphone for all your other genealogical needs as well, e.g.: to record video interviews, take down all your notes, provide translations, display your genealogy, access archive information, take pictures of 3-D artifacts that can't be scanned, and take pictures of the home towns, residences, schools, workplaces and burial places of your ancestors. All your photos will have embedded GPS coordinates, so you'll be able to plot their locations on a map. No scanner can do all this.
A digital camera will do no damage to a fragile document, other than what you must do to hold the item open for the picture. A scanner always requires some extra form of restraint on the item, and that always adds a possibility that some damage may occur.
I still use my flatbed 1-or-2 sided scanner/copier with auto sheet feeder for standard 8.5 x 11 inch paper and photographs, but never for anything else, and especially not for fragile documents.
But I have used a digital camera, and more recently my smartphone as my personal portable always-with-me document scanner with great success. To me it is perfect way to quickly and sufficiently record the genealogical media I get from relatives and other sources.