What about transparency adapters on a flatbed?
These adapters are usually good for doing big negatives or slides - medium format or larger. The sensors, though, just cannot get enough information from the small area of a 35mm slide or negative. If you are going to be doing a great deal of flat, reflective art and a few of transparencies or negatives - especially if these are only for web or screen work - you will probably be fine with such an adapter. If you do go this route, shoot for as much resolution as you can get - 1000 X 2000 would be nice. If your work will require you to scan 35mm negatives or slides often, or if you will need big files from this little images, get a good 35mm film scanner.

Other buying guides
Review Amazon.com's guidelines on buying a scanner for more help on the subject.