ResolutionFirst, one of the first features you will see emphasized is resolution, given in dpi or dots per inch. The resolution limits of a scanner tell you how powerful the scanner is at replicate the detail of the item that it is scanning, but there is an important catch. When scanning something like a photo, there is a limit to the amount of detail that can be captured in the first place.
Now, if you plan to scan film, slides or negatives then a higher resolution is important. That is because a negative (or slide) has a much greater resolution within itself to be captured than a print made from the same negative. The photo print has essentially lost detail to the printing process (as photo printers have a max resolution at which they can print).
So if you only plan to scan photo prints then you do not need to necessarily purchase the scanner with the highest resolution. But if you think you may one day want to scan film or objects, then go for more resolution.
See a list of the top photo scanners here:
Image EnhancementAnother thing to compare when trying to choose a photo scanner is the image-enhancing functions. For example, the Canon 9000F has a feature called FARE which will automatically correct for defects in your photos like scratches or color fading. Some of the mid to upper level Epson scanners have a similar technology called Digital Ice. These functions can improve the quality of your scans without much intervention on your part, but they also come at a higher cost.
Scanning SoftwareSoftware packages are another thing that can differ from one scanner to another. The cheapest photo scanners do not include photo-editing software like Photoshop Elements that some of the best scanners have. If you would like to do some creative editing of your photos, then the addition of these programs can sometimes be more cost effective than buying them separately so keep that in mind.
Finally, don't forget to check the dimensions of the scanners you are considering. Flatbed scanners have a larger footprint than some people realize and you need to make sure you have the space to set them up. If space is a major issue, there are also portable photo scanners available which are very compact. Image quality will not be as good, but if you have nothing fancy planned for your photo images then that may not matter anyway.