Wacom is a name everyone knows in tablets, but expensive (though they do have some cheper models).
AIPTEK are the brand behind many own-brand products - Serif sell a rebadged Aiptek tablet.
The Wacom, does have some additional download tools, that can add features to some software, but the main comparison points are:
1. Size & resolution - depending on the scale you work to, you may prefer a smaller, high resolution, to a larger, lower resolution.
2. Pressure levels - useful for stroke emphasis, but unsupported in many cheaper programs.
3. Tilt - some also sense the pen tilt, though it's a difficult effect to use.
4. Pen features, the most seems to be pressure, a 2 way click bar, and an eraser end.
5. Mouse - a great space saver, though you CAN use most tablets with a normal PC Mouse, so long as you do not leave the pen on the active area of the tablet.