Except when very high quality copies of photos are needed, scan quality is about the same for all makes and models, but speed varies considerably. Epson Perfections are among the fastest, UMAXes the slowest. The only other major hardware difference is that some scanners use CCD optics (fluorescent, "cold cathode" light source) while others use CIS (LEDs for light - instant warm-up, half the power consumption), and CIS optics can't focus over a range greater than about 1/8", something to consider if you need to scan non-flat images, like pages from thick books.
I wouldn't buy UMAX unless it was very cheap, and HP isn't worth the price, especially when the warranty is only 90 days (cheaper models). Visioneer and Agfa seem to have good cheap models.