Some concerns about scanner features are:
Computer Operation System
Make sure the scanner provides drivers for your computer's operating system version, for example Windows XP, or Macintosh OS X.
Flatbed or Film Scanner
Having one of each is a very desirable choice. Flatbed scanners are the best and most versatile choice for photo prints and documents, and they dominate the market now. Older choices included sheet-feeder scanners with page rollers, still seen in some low-end multi-function (fax) units. Hand-held scanners have vanished, the results were not so good. A good digital camera with a closeup lens may be a better handheld scanner (tripod recommended).
The flatbed can scan pages while still attached in the book or magazine, and it can scan portions of documents larger than the scanner bed (like maps). Most scan 8.5 x 11.7 inches (including metric A4 size). A few handle 14 inch legal size documents, and a few are A3 size with a 12x17 inch bed. The table space required is the only disadvantage of a flatbed. The big advantage of a flatbed is that the original lays motionless on the glass bed, it won't move between preview and scan, and there is no risk from any feed rollers. Some have an ADF option (Automatic Document Feeder, for documents, not for photo prints), and some have a Transparent Media Adapter option (TMA) for scanning film or even old glass negatives. However, flatbeds just can't do the tiny 35 mm film size well due to insufficient resolution, and inexpensive scanners don't have the dynamic range to do film well.
A film scanner is extremely desirable for 35 mm film (see more about film scanners). Film is the first generation original, and the best results come from the original film. The 35 mm film scanners generally provide 2700 to 4000 dpi. 2700 dpi allows printing 8x10 inch prints (9x enlargement of 35 mm film), and 4000 dpi allows even larger prints. There are several 35 mm scanners of various prices, but only three for medium size film, and there are three for 4x5 inch film. A high quality flatbed with TMA can also be used for larger film. The top end units have advantage of sufficient dynamic range for scanning film in a critical way. Your goal may allow accepting less, but dynamic range is an extremely important feature for scanning slides, the more the better (but see more about dynamic range). Scanning negatives has lesser equipment requirements than slides.
To scan film on a flatbed, at minimum there should be a lamp built into the flatbed's lid, with the lighted area at a fixed location to allow CCD calibration from that upper lamp. The scanner's firmware knows how turn off the lower lamp and to calibrate the CCD from the upper lamp. The scanner software should have modes for both slides and negatives (see more about flatbed TMA). But a $200 flatbed is not really a film scanner, and 1200 dpi is insufficient scanning resolution to print 35 mm film full page size. Best results are obtained from a real film scanner. Frankly, while scanning film can give the best results, it does require better equipment, and on an inexpensive flatbed, scanning prints is often the better plan.
Don't get too excited about the separate free 35 mm slide adapters that come with some flatbeds. Customers expect them to actually work, but they may be disappointed in results. They may be better than no image at all, perhaps "fair" but not "good" results. The flatbeds may be great for prints and documents, that is not an issue, and better flatbeds can do film well too, but the free film adapters (simple lamp or mirrors) are not a serious tool. A good toy, more like the prize in the cereal box, and if you see it that way, you may be satisfied for casual results, but these free gadgets are not serious tools.
If you want to print 35 mm slides or negatives to be photo quality at 8x10 inch or A4 or larger, you'll really like a real film scanner, at 2700 dpi or more. These scanners are more expensive, and a bit harder to use (because film exposure varies, but photo prints have already been corrected), but can give substantially better large images than any flatbed. You will surely also want a flatbed for print and document work.